I feel like 8 is already a good number of companions. I want them to have depth, and the more they try to add, the less depth they will have.
This is of course subjective, but as much as I agree with quality>quantity, I think Larian went too far. Choice is important too, especially in a cRPG. BG2 companions were very well developed and certainly had depth.(Most of them, anyway.) At the same time, you could assemble a party according to whatever you wanted: there were 15+ companions to pick from if you're very picky and wanted the ones you like; party comp options; good parties, evil parties; serious parties, funny parties. I consider BG2 depth to choice ratio a very good balance. It improved upon BG1's multitude of flat companions, but imo it would be best to stop at BG2's proportions.
