Hi there!
The game does indeed requires a GM. The game is designed for any number of players, but really excels at 3-4.
One may need to adjust things with less or more players: challengers and combats tend to be solved faster the bigger the group is, but there is a faster suggestion of Drama. A group of six or more is bond to always be putting fires in their character's lives; a group of one GM and one player may find combat risky and spend a lot of time each challenge, but has a slower, more focused story on their own drama.