It does require a GM.
The tactical combat is best with 3-5 player characters. Fewer than 3 works, but is more limited in the types of combats you can present. I've played with 2 players and just had each one control 2 characters in combat, and we had a blast. More than 5 player characters and things start to slow down a bit too much for my taste, but it's not like anything stops working - it's just that with so many other players taking turns, you end up waiting around a while for your turn.