I just want 5 for diversity sake in my party, could roll with 6 though, Four works at a base level by my estimation but gives me no variety for classes as I always cover my base roles (Tank, Healer, Caster, Rogue) and while yeah maybe I could make the rogue a bard and get a little extra mileage out of 4 that way I find I prefer a 5th slot for something that can be fun without having to cover a core role. Six would give me alot more variety but might seem a bit cumbersome for my personal preference. But I would role with 6 if that was the party size.
But Four for me is basic setup.
Less than 4 and I feel like I am missing key components.
And while I fully well expect someone to mod a 5+ party and WILL use the mod. I would prefer Larian to code a 5+ party to avoid or at least cause less errors and code conflicts.