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
Well, a GOOD argument for four would go a long way. Too bad there can't be anything else than half-hearted bullshit arguments for it, because we aren't talking about guessing randomly and taking a stab in the dark, here.
People who want a six-men party already played games that had them, have solid reasons to think it's better than the alternative and have tested their expectations in reality over and over across the years.
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.