I'm doing the main quest and gathering allies for Bruma but I'm getting tired of running through Oblivion gates.
How many allies do I really need for the next quest?

you actually need none. but having them makes it easier to battle the daedra. you know, more cannonflodder

I know they aren't necessary but do they make the next quest much easier?

That is correct. Because in the next quest you need to:



And the more allies the better. This is one of the few times in the game when allies actually help.

If you only get two allies, I recommend Cheydinhal (assuming you didn't do the Corruption and Consequence quest yet) and Leyawiin. If you didn't do that quest in Cheydinhal yet then the ally is essential and cannot die on the battel field. The named ally from Leyawiin maybe be stronger than some of the generic guards sent from other places. However all allies help, even though it can get tedious to gather them.

Alrighty, thanks. I better go gather some allies.