You only need two or three in your party in most circumstances as a rogue can solo significant parts of the game (as of now) and the rest are just meatbags to slow the enemies down or offer targets for an AI which does not know how to prioritize. You need their passives for some checks or a rogue if you are not one yourself. I guess your character, a rogue and a caster for surfaces, speaking with animals and a bit of cc could take down most of the game with patience.

Solo kill list: One stupid evil (displayed alignment) Githyanki warrior, a dark grove, a Zhentarim hideout full of "experienced" rogues, half a Goblin camp including two hobgoblins, two minotaurs, a bunch of underworld monsters and their wannabe sorcerous overlord