My group began a new campaign recently and it turns out four of us rolled rogues (me included). We also have a nature cleric and sun soul monk. It's written into my characters backstory that I'm going to multiclass with wizard later on, around 8th level (12 levels of wizard by the end hopefully). I had bladesinger in mind but after a bit of thought maybe something more supportive to our impressive, albeit squishy, dps. What are your suggestions for subclass and spells to support and get the most out of the many rogues in the party

The key to Rogues in combat is getting them Sneak Attack as often as possible. Anything that can give them advantage is perfect. Blindness on their enemy for example. Anything that can knock a creature prone would be useful too.

Conversely - because they're a melee heavy group - anything that can stop the enemy stomping on them and/or make the enemy easier to hit would be great. Slow comes to mind for this.

Necromancer. That way you can quickly replace all those dead rogues.

I'm kidding.

(Mostly...)

Maybe abjuration for arcane ward? Necromancer actually could fill the ranks to give enemies more (non-rogue) targets. Conjuration with let you switch out with an injured rogue or quickly snap the heavily armored cleric to the front lines. Divination's portent can be useful defensively (but a bit random).

I think spells are going to be more important honestly. Abjuration is the closest thing to health boosts wizards can get, and some summoning spells to put up meat walls. Polymorph can turn a rogue into a meat wall (practically a healing spell and a buff combined).

Hold Person/Monster are the best spells to help your Rogue because it gives their attacks an auto-crit if made within 5 ft thanks to inducing the Paralyzed condition. That auto-crit means their damage die get doubled in number: including their sneak attack which they will automatically get too as it grants advantage to attack.

Greater Invisibility is great to cast on your Rogue, so they always attack with advantage getting sneak attack and helps them stay safe too. 

Abjuration wizards can help protect the squishy rogue. Divination wizards can help the Rogue hit or might make an attack against them miss and they can use scout spells like Arcane Eye or Clairvoyance to plan out the best routes for the Rogue to stealthily sneak in and attack/steal from (useful if Assassin who kinda need to go in alone first to make use of Surprise). 

There's a LOT of ways a Wizard can help their Rogue, frankly.

Thanks for the replys everybody! Next question, is going the seven (prolly eight for the ability score improvement) worth the less powerful spell slots? If not are more powerful spells worth going down to level four rouge and missing evasion?

Edit: just thought also, is going only two levels in rouge and getting spell mastery eventually worth losing the ability score improvement? At that point I'm guessing going straight wizard is a better idea

I’m not sure which school to pick, but there are so many great transmutation spells that can help rogues. Longstrider, Jump, Darkvision, Knock, Spider Climb, and Levitate. And that’s only 2nd level spells!

Quote from Thorzap >>

Thanks for the replys everybody! Next question, is going the seven (prolly eight for the ability score improvement) worth the less powerful spell slots? If not are more powerful spells worth going down to level four rouge and missing evasion?

Edit: just thought also, is going only two levels in rouge and getting spell mastery eventually worth losing the ability score improvement? At that point I'm guessing going straight wizard is a better idea

 Whenever a build has the opportunity to get wish, I encourage it. Evasion (and dodge for low HP class) could be good to have, as is reliable talent at level 11 (even makes up for the lost ASI at level 10).

I based my recommendations assuming at least 6 levels wizard. You can go more. You certainly have enough rogues to do rogue things.

Wizard can always benefit from cunning action and light armor. (Skills and expertise ain't bad either). Spell mastery is pretty good, but I recommend 3 levels rogue for archetype, your cantrips will be pretty powerful by this point and you will have a good number of slots.

You might want to look at Rouge to level 4 for the stat bump/feat.  Arcane trickster can be a good choice to get cantrips from 2 different sources.  Also it counts at a 1/3 rate of levels to increase your spell slots.

Update:

Yesterday was another session and I decided to take next level in wizard for some early support spells. Following some suggestions on what spells to take this character proved invaluable! We were faced with defending a cow (yup) from a large army of Kobolds (around 40 or so). We had a day to prepare a farmstead and an army of halfing villagers to help us prepare (but not fight). With some careful planning with traps the traps easily took out half of them. But with spells like sleep the flying Kobolds fell to their death mid air! When a scale sorcerer cake out of the treeline and charmed our monk I used my own Charm Person spell to free him! It was amazing! As someone who has never played a spell caster focused on "support" it really is a shocker just how one person focused on this can change the tide of a battle. Thank you all for your suggestions!

Glad you enjoyed it.

Don't forget you can cast Mage Armor on others. Including cows.

Quote from NordicNinja >>

Don't forget you can cast Mage Armor on others. Including cows.

 Unless they're battle cattle wearing armor.  But yes, that's an excellent suggestion for any time you have to act as bodyguard for something unarmored.

Also, side note.... 4th level Arcane Trickster Rogue with 16 levels of Wizard gets you 8th level spells and a 9th level spell slot.  And while you can't learn a 9th level spell when you hit 20, the rules are a little fuzzy on if you could ad a 9th level spell to your book to cast after the fact, so a GM in favor of giving you a 9th level spell could rule that you could learn one through study as like a quest reward sort of deal.

Quote from Arhra >>

Also, side note.... 4th level Arcane Trickster Rogue with 16 levels of Wizard gets you 8th level spells and a 9th level spell slot.  And while you can't learn a 9th level spell when you hit 20, the rules are a little fuzzy on if you could ad a 9th level spell to your book to cast after the fact, so a GM in favor of giving you a 9th level spell could rule that you could learn one through study as like a quest reward sort of deal.

The rules for adding spells to your book are quite clear. You can only add spells to your spellbook if they are of a level you can prepare. Using multiclass rules when preparing spells for each of your classes - you take each class as if you were single classed. So for a level 16 Wizard you cannot prepare 9th level spells and so cannot add them to your book.

Wizard:
Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.

Multiclass Spellcasting:
Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

Of course that doesn’t mean your DM can’t let you have one if that’s what they want to do - but it wouldn’t be RAW.

Edit: Added quotes.

Emmote covers the rules.

Imagine if a wizard could add spells to their spellbook regardless of Wizard level. A level 1 wizard level 19 cleric would have all the most powerful attack spells and healing spells. It would have the spell list of 17 levels in 2 classes. That is why the rules don't allow it.

There's an argument to be made for Evocation Wizards here. With all those rogues, it might be a melee heavy party, yes? Sculpt Spell will really save some lives over time by letting you drop big AoE spells in the midst of the fracas without worrying about whether your allies are vulnerable to it or not. 

The Enchantment Wizard's Hypnotic Gaze may also be useful; it lets you incapacitate a single creature, which could make quick work of any boss fight.