You can mix and match roles as much as you'd like, as long as you keep a few basic concepts in mind:

1. Include more Villagers than Werewolves and the Minion combined.

2. Always have a Seer or Fortune Teller, and at least one Werewolf.

3. Don't change roles out between every game. Add or subtract different roles frugally; switching things around too fast can be confusing for players who have to try to remember what roles are in the game.

Here are a few good sample rolesets at different player counts:

4 Players

6 Players

9 Players

12 Players

18 Players