Im building my gameworld, im pretty new to dnd and i like the game a lot. Now, i wanted to have an Adventurers Guild in the game world, that players can join, and get missions to go explore dungeons, ruins, dangerous areas etc. So there are supposed to be a good amount of other adventurers in the guild you can interact with, exchange info with, or maybe cordinate attacks on a mission or so.
My question is however, why adventurers would not go to dungeons in really large groups? Like, the in-world reason. Not gameplay wise, yeah i can just put bigger badies in their way if they drag along 5 extra adventurers. But i'd like it if there is a good in-world reason to why people normally do this in smaller groups.

So I haven't figured out a good reason for this. Food and Water seem to be quite easy to provide for the party with first level spells, so that is not a problem at least when it comes to travel. I could see traveling into another country would be a bit of a bad idea with a large group, the local athorities might not like that, but many adventuring places is not within other countries borders in my setting (altho some are). Would enemy scouts have a better chance of spotting a larger party? Marginally only imo. I can see adventurers not wanting to split the reward between to many people, but considering how dangerous adventuring can be, it seems worth it to me at least. Maybe it is a trust thing, people mostly go on these types of mission with those they trust, altho good reputation can prevent that somewhat. Maybe it is the prestige of clearing a dungeon, makes for much better stories.

Seems like i got a few ideas when writing this post, but now that i have, i might as well see if others have good ideas for this. So why would people go adventuring in small parties of 3-5 people most of the time? And why is 20 men dungeon raids not a thing? Why does the army not engage in this type of activity?

Unless there's a very strict chain of command and standardized training within the guild, trying to coordinate that many people is a pain in the ass. A large group has a hard time being stealthy. A large group can't fight effectively in cramped spaces. If any part of the dungeon requires special movement modes, now you've got to figure out a way to move a heck of a lot more bodies. There's more people that could set off traps and more people that could get injured by traps. Different groups might not be familiar with each others' tactics and step on each other's toes. You might get disagreements when it comes to making decisions or handling traps, puzzles or combat tactics.

You'd only want to send that many people in if you've already got intel on the dungeon, a clear plan of action and know there's a monster in there that requires that many men to kill. But adventurers often venturing into the unknown and it'd have to be a heck of a monster for you to need a group of 20 high-level adventurers.

It'd be more efficient for the guild to dispatch smaller groups to multiple places simultaneously.

Most dungeons have pretty narrow corridors.

A large group will set off more traps and attract more monsters. And monsters are not so stupid that they will attack one at a time against a large group. Because a monster is more likely to notice a large group quicker, it will retreat until it meets up with other monsters and they can ambush the group at the best opportunity.

Large groups are also harder to coordinate and move slower. This will make progress slower and tactical retreating very difficult. This will be amplified by the typically small size of dungeon corridors (20 adventures mean nothing if only 2 can fight the monster in front of them).

You also have to consider that adventurers have unknown loyalty, so trust and authority will get called into question frequently, and there will be thieves, deserters, and infighting.

Ultimately, these factors will add up and result in a higher death count than if the same number of people had gone in smaller groups.

Thanks for the ideas. Got me thinking it could be kind of interesting to explore what happens if a rival adventuring party is at the same dungeon as the PC's, see how they would interact with the rivals. Maybe these types of mishaps have been a thing in the past, so the guild try to keep missions secret, handing them out to who they deem most fit for the mission, so only one party at a time goes on that adventure. Sometimes the information get out however, or thievs keep track on adventurers in order to try and steal the loot when convinient.

Quote from Brynhold >>

Thanks for the ideas. Got me thinking it could be kind of interesting to explore what happens if a rival adventuring party is at the same dungeon as the PC's, see how they would interact with the rivals. Maybe these types of mishaps have been a thing in the past, so the guild try to keep missions secret, handing them out to who they deem most fit for the mission, so only one party at a time goes on that adventure. Sometimes the information get out however, or thievs keep track on adventurers in order to try and steal the loot when convinient.

Could be fun to give your players a recurring rival team who always seem to show up to wreck their shit and all that.

Bigger team: more people to split the same amount of loot with!

The kind of adventurers who show up to raid a dungeon full of monsters to get the gold inside are definitely going to want to take the smallest group they think they can get away with, fewer people to share with.

Quote from ftl >>

Bigger team: more people to split the same amount of loot with!

The kind of adventurers who show up to raid a dungeon full of monsters to get the gold inside are definitely going to want to take the smallest group they think they can get away with, fewer people to share with.

 Also people that engage in more dangerous fights will get stronger faster, at least if exp works for everyone the way it works for the PCs.

Most "quest givers" are not looking to employ an army.

There is also a huge level of trust issues. The PCs are the new kids on the block, no group of veteran guild members are going to want them tagging along. Parties are usually close knit groups, they're not going to trust some stranger to watch their back or stand with them in the face of danger.

Regarding the army, that would be dumb. It would be a huge waste of money and manpower. On top of the fact that they're probably lvl 0-1 and would get crushed by mid level dungeons. Losing tens of guys for no gain is bad for morale.