I'm content with 4 companions, someone at Reddit summed it up nicely, though judging by responses there is lots of die hard fans of 6 members and no matter arguments everyone against them will hit a wall so why bother. 4 that's the number
So when the "modern game designer" who watched five episodes of Gamemaker's Toolkit on youtube comes in, winks at you and tell you knowingly "Trust me, you don't really want a party of six, that would mess up TEH PERFECT BALANCE" he can't really expect anything more than being welcomed with loud burps.
Story balance / interaction. It’s why I initially said 4 core plus extras up to 6. If you have a party of custom characters, less issue (and a whole other subject), but 6 origin characters all interacting the way I believe they are intended to, that potentially creates a workload headache Larian would prefer not to touch.
Again, you could limit it and say max 4 origin characters, though I hope for more work on customs so that they too have more interaction, or 2 mercs or whatever, but that’s the reason for 4 as I see it.
Larian know how to get 4 to work from experience, anything likely creates a timeline issue.
Just thinking out loud, I obviously don’t know either way, just trying to answer your question!