Like other traditionally-structured adventure RPGs, it could work with 2 up to a very large number of players. I think it works best with 3-5 players (including the game master) but maybe handles 1-on-1 play better than some other trad games.
It’s built for campaign play, which could be any number or sessions lasting 1-4 hours each. You probably need 2.5-4 hours for a satisfying one-shot featuring multiple characters.