I think 6-8 possible companions is a solid number, spread out across various classes. I'd even say that for me it's about the sweet spot I don't think I've ever played a game that gives you as many as 13 party members and while I'm sure that can be good, to me it almost sounds kind of overwhelming. And as far as the number of characters with you in your party, I'd say that while 5-6 would be my ideal purely because it allows for more interaction with your party members and it ensures you'll be able to mix and match your party without too much worry, if the game is balanced for only 4 characters then I'm sure it'll work out fine. The studio knows what it's doing.