Swen said in the video that by default it allows 4 + 1 GM, but potentially could be modded for more. I recall them saying something a long time ago about D:OS1 that they had 8 players in one game and it worked fine (except the UI doesn't really work for that.)
Probably you would have to modify the player UI to be slightly smaller to fit 6 players. It'd probably get pretty unwieldy with 7+ but who knows what the actual technical/performance limits are.
