The base game supports up to 5 players. However, if you add other Rangers from the expansion or stretch goals, you can play with up to 6 players! Any combination of up to 6 Rangers can be played together. So if you want to mix teams, you can do that.
Also, you can play with more than one Ranger per player. For example, in a 3 player game, you can each choose two Rangers and play a game with 3 players and six Rangers!