Wasn't there 6 in BG one & two?
There was.
There was a six-men party in he overwhelming majority of titles in this genre too.
Icewind Dale 1 and 2, Temple of Elemental Evil, Pillars of Eternity, Planescape Torment, etc.
But somehow now we need to pretend it would be an "incredibly complex setup that would take too much work to function properly" or something.
