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.
Not really sure what you are exactly trying to say, but let's be clear of one thing upfront: wanting a six-members party has nothing to do with the idea that "you NEED it" to beat the game. It's about enjoying the variety of characters, builds and possible party compositions that comes with it a lot more.
Not to mention being able to carry on more companion questlines during a campaign.