This game can technically support as many players as you want, but comfortably 1-4 or 5. You can play it solo, and I know some who have, but it's simple nature of answering questions is intended as seeds to discuss and expand on with others to riff off!
Game time is also variable, since the "goal" is to get discussions going and ideas flowing! I've used individual tables to play a quick 20 minute game at convention lobbies and also had whole 3 hour worldbuilding brainstorms going thru all the tables available!