The engine currently supports 6 players, but the game only supports up to 4.
For single player, having so much dialog for one non-lead party member should add a lot to the game, but after that having more people chime in for every decision would start to become a problem. With multi-player, 2 player co-op would cover the majority of cases, with fewer people having 2 or 3 friends they would want to play with and could coordinate playtimes.
Each additional party member added requires more work to create unique dialog and interactions, while the benefit for most gamers goes down.
