It´s confirmed that BG3 would have 4 player coop multiplayer. I do not know if that could change in SP.
The standard party size in D&D is 4 players. (Standard as: this is the size WOTC makes his campaigns for. You usually have to tweak it later)
Right now there are only 5 origin companions that we know of, and that would be too few for a 6-man-party, but that could change.
