The main game is focused on 2 player co-op. Mods will be able to have 4 players, maybe 6; main game has 2 PCs and up to 2 henchmen / companions and 2 summons (plus the possibility of an NPC joining the party temporarily), but I don't recall if players can take over the 'summons' slots (don't think so). The engine itself supports more players, but the current UI, etc, does not expose that.