So one of my players wants to, over many sessions, casting the awaken spell on an army's worth of animals, like deers, squirrels, dire wolves, bears, etc. His plan is to mount crossbows on the deers antlers, which will be fired and reloaded by the squirrels. The bears and dire wolves will be having platforms on their backs and battering rams or something to plow through enemies. The platforms will have multiple users. Either they will carry melee units in or out of battle, they will carry ranged units, or they will have crossbows or similar weapons on the platform that the squirrels will fire. Another was he will awaken birds, like eagles or hawks, and have them carry platforms over the enemy army which ranged units can fire from or they will carry explosives or similar weapons into battle to drop in the midst of the fight. All this sounds good, especially since they will have a huge fight with an enemy army attacking Baldur's Gate, but is it too insane or OP?

I think it’s good and the use of the spell. but they will need to get a lot of the expensive material component

Fun idea! But it is going to be a logistical nightmare to actually play, unless you figure out some kind of method for large scale combat.

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So one of my players wants to, over many sessions, casting the awaken spell on an army's worth of animals, like deers, squirrels, dire wolves, bears, etc. His plan is to mount crossbows on the deers antlers, which will be fired and reloaded by the squirrels. The bears and dire wolves will be having platforms on their backs and battering rams or something to plow through enemies. The platforms will have multiple users. Either they will carry melee units in or out of battle, they will carry ranged units, or they will have crossbows or similar weapons on the platform that the squirrels will fire. Another was he will awaken birds, like eagles or hawks, and have them carry platforms over the enemy army which ranged units can fire from or they will carry explosives or similar weapons into battle to drop in the midst of the fight. All this sounds good, especially since they will have a huge fight with an enemy army attacking Baldur's Gate, but is it too insane or OP?

 Depends, how do you intend to allow them to be used? Any PC should be able to recruit and army in various different ways - a cleric or paladin or monk could call on their church / holy order / temple / monastery for aid, a fighter could call on the army / a king for aid, a rogue might call on the city watch or a theives guild for aid, etc.. etc... If you intend for there to be an army vs army battle in the background then I see no issue with allowing the druid to use Animal Friendship or Awaken to create their portion of the army. But raising an army should always be for a specific narrative purpose not a "just because I can" situation. However, allowing them to bring along with the rest of the party 5-6 awakened animal soldiers or regular quests is a big no-no, it will be a nightmare to play at the table and make the rest of the players super bored during combat while they wait ages for their turn.

First, as an above poster notes, it’s going to cost 1,000 per spell in material components. That’s assuming he can find enough agates. And with an 8-hour casting time, he can only possibly awaken 2 animals per day, since he’ll need to sleep. 
And if this process takes a long time, after 30 days, the awakened animals will have a chance to opt out of working for the party.
After that, rule of cool would be to ok it and not think about it too much. But if you do stop to think, there are some problems, like a squirrel is simply not strong enough to load a crossbow, that is to pull back the bowstring. And neither the squirrels nor the deer would have proficiency in using crossbows. And while a brown bear is pretty strong at a 17 str, it is not realistically going to carry more than 1 or two other creatures (plus some kind of platform? Why not just a saddle?) based on weight alone. And how many bears and/or dire wolves are there close enough to the city to even be recruited? And what sort of ranged units are small enough for an eagle to be able to carry?

And, really, if you’ve got a bunch of awakened bears and dire wolves willing to fight for you, you just send them in. No need to weigh them down with platforms and rams and other creatures; they’re both going to be pretty effective against the low-CR NPC types who make up most armies. And that’s to say nothing of the psychological effects of seeing a dozen plus bears charging at you.

If you do allow it, you should definitely put in the next campaign some squirrel/deer combo that’s gone rogue and started raising a bandit army of their own to liberate all the animals from humanoid tyranny.

To answer y'alls "it's expensive" thing, my players have defeated a Blue Greatwyrm and then stole its entire hoard, getting around 1.6 million gold. Money isn't really a problem and it shouldn't ever be. (There's supposed to be a heist side quest for them soon, where they can get more gold and the same player who had the army idea wants to buy nearby taverns with a kinda ingenious idea to get some constant income) To answer Agilemind, no they won't be allowed to have these awakened animals as constant followers (My party has enough NPCs). He is an elf, so he has the trance ability. Xalthu, yes the next campaign will have rampaging intelligent animals seeking freedom.

Generally speaking I love creative stuff like this however as others have mentioned I see 2 main issues - if you rule that he can acquire 50 animals. Now are you then going to do 50 dice rolls and have the enemies roll 50 to hits as I assume the enemy would need numbers to match the incoming hoard? What are the other players doing all this besides being bored?

If I allowed it and I likely wouldn't only because it smacks a little of main character syndrome - I would say your army goes off and fights a division of their army to  a stand still - back to regular combat order - and proceed with combat for all the real players at the table. 

A good rule of thumb is there should never be more 1/2 the dice rolling for NPC's than there are for players (Not including enemies ofc). The players get bored and it makes the session not fun. So 5-6 players = 2-3 meaningful npc's max. 

I have been in games with lots of NPC's trust me its NOT fun for any of the other players. 

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To answer y'alls "it's expensive" thing, my players have defeated a Blue Greatwyrm and then stole its entire hoard, getting around 1.6 million gold. Money isn't really a problem and it shouldn't ever be. (There's supposed to be a heist side quest for them soon, where they can get more gold and the same player who had the army idea wants to buy nearby taverns with a kinda ingenious idea to get some constant income) To answer Agilemind, no they won't be allowed to have these awakened animals as constant followers (My party has enough NPCs). He is an elf, so he has the trance ability. Xalthu, yes the next campaign will have rampaging intelligent animals seeking freedom.

Even with the trance, he needs the rest. If you allow him to get the benefits of an 8-hour rest in four hours, it still only leaves time for 2 castings in a 24-hour day. He'll just have an extra 4 hours, which will be nice, so he can get a break in there from his 16-hour-a-day job in the awakening sweatshop.

If you don't want money to be an issue, then it isn't. That's fine, of course. There's still the question of the availability of these gems, which, again, is only an issue if you want it to be. I think our point is more, you can use that as a brake if you want to tone down the plans, if you want.

You lost me at deer antlers.

It'd be fine; it's the player expending his/her own resources. The problem is he combat mechanics. Mass combat in Dnd absolutely SUCKS and bogs down the game. A lot. You could run them as swarms and siege weapons, I guess.

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To answer y'alls "it's expensive" thing, my players have defeated a Blue Greatwyrm and then stole its entire hoard, getting around 1.6 million gold. Money isn't really a problem and it shouldn't ever be. (There's supposed to be a heist side quest for them soon, where they can get more gold and the same player who had the army idea wants to buy nearby taverns with a kinda ingenious idea to get some constant income) To answer Agilemind, no they won't be allowed to have these awakened animals as constant followers (My party has enough NPCs). He is an elf, so he has the trance ability. Xalthu, yes the next campaign will have rampaging intelligent animals seeking freedom.

 That sounds like a logistical nightmare. Assuming the wealth was all coinage, that's somewhere in the neighborhood of 16 tons. Granted, that would be absure. Hoards are also supposed to include gems, art objects, and magic items. But you aren't breaking all that down for one druid's pipedream.

But I digress. The actual expected coinage, as presented in Fizban's, is only 20,101 in gold pieces and ≈21,000 in gems; though both the prescribed gem tables in the DMG lack agates. And all of that could fit into an empty Bag of Holding, so it's not impossible. It's just time-consuming. And even if they rolled high, like maxed every die roll on the table, that isn't even 375,000 worth of gold. Split five ways, that's 75,000 gold a piece, but there's still 1.215 million in gold worth of magic items unaccounted for. That's multiple legendary magic items after defeating a Blue Greatwyrm. That's endgame territory.

Just how much more can the players do, and what are 50 animals that only owe their allegiance for 30 days expected to accomplish that they can't do on their own? Or is this some epilogue, "I'm going to make an army of woodland critters," shenanigans?

If it were me, I’d allow it, but I’d have a talk with him first.  Awaken is a divine or primal spell.  I’d like to know how he sees this promoting the cause of his god / nature.

With the mass combat, I would say that the animal army goes to a part of the enemy army, while the players go to a different location. Though I have a question. What about using the side initiative in the 2014 DM's Guide? Would that make it easier or would it just make it more painful. To answerJounichi1983's mentions, my players are fifth level and they got real lucky with the Greatwyrm. The bag of holding I run is the infinite space one because that one just makes more sense (they only have one, will only get one, and its communal). The animal thing is just supposed to be a big "I got power now" plan. He wants to have the army constantly get larger over the next who knows how long. He plans to actually win their allegiance at the end of the thirty days.

Quote from IMPLAYS >>

With the mass combat, I would say that the animal army goes to a part of the enemy army, while the players go to a different location. Though I have a question. What about using the side initiative in the 2014 DM's Guide? Would that make it easier or would it just make it more painful. To answerJounichi1983's mentions, my players are fifth level and they got real lucky with the Greatwyrm. The bag of holding I run is the infinite space one because that one just makes more sense (they only have one, will only get one, and its communal). The animal thing is just supposed to be a big "I got power now" plan. He wants to have the army constantly get larger over the next who knows how long. He plans to actually win their allegiance at the end of the thirty days.

 Sorry, I don't think our advice is at all relevant to your game. A Greatwym should TPK a party of 5th level characters with a single breath attack, and level 5 characters shouldn't be able to cast Awaken at all since it is a 5th level spell (requires 9th level to cast). So you do you.

If it fits the story of your campaign, and isn't just a ploy to make the group "unstoppable", then go for it.

Oh no, he doesn't have awaken yet, he plans on getting it once he can. And the greatwyrm (I admit this was probably a horrid decision, but I already let it happen. Regrets) died by swallowing a bottle of Abyssal Mist, which choked him and rotted him from the inside. 

I'm going to second Agilemind. The game you're running has already broken so many expectations that I do not believe we have meaningful advice to offer you.

Short answer is probably not. However, since you run a rather unrealistic game in the first place, it may not matter, in which case just decide what you want to do. It IS a cute idea.

Here is the text for Awaken (2014) version:

"After spending the casting time tracing magical pathways within a precious gemstone, you touch a Huge or smaller beast or plant. The target must have either no Intelligence score or an Intelligence of 3 or less. The target gains an Intelligence of 10. The target also gains the ability to speak one language you know. If the target is a plant, it gains the ability to move its limbs, roots, vines, creepers, and so forth, and it gains senses similar to a human's. Your GM chooses statistics appropriate for the awakened plant, such as the statistics for the awakened shrub or the awakened tree.

It really depends a lot on how you see the fantasy of the game and whether you are make considerations like real world physics, intelligence level of monsters and other aesthetic things that can of coursed always be explained in a fantasy setting as "magic".  Point is that this isn't really a question of "rules", its more of a question of theme, style and more abstract narrative concepts.

Its basically a question of what sort of fantasy game are you running?

Well, if you have a huge fight coming up, this sounds like it'd be pretty epic. If they can get the spell components and spend the spell slots to do it, sure! For the sake of simplifying things, I might make a comprimise of using or making some custom stat blocks for these these awakened creatures with weaponry. Or maybe put a cap on the maximum amount of these creatures they can have to keep it from getting too out of hand or unbalanced. I have no idea what you plan on throwing at them. And these are just some common sense logistical observations on my part, but why not just awaken actual rams instead of bears/dire wolves with battering rams? Not to mention, some of these animals would be predator and prey. That might be a confusing alliance fo forge. I'm not that familiar with the awaken spell, so maybe that wouldn't be an issue, but does that spell keep the animals awoken afterwards, assuming they survive? For that matter, would the animals even want to fight for their cause? I'm probably overthinking it, but hey, that's what I do best.