For example, in a twenty player game, if the town is all vanilla, it might be balanced with only 3-4 scum. If playing in a twenty player game with many different power roles, then two scum groups of three people each might be balanced. No matter how many power roles, ideally the mod should be able to balance the game in such a way so that it doesn't matter how many power roles there are and still retain a resonable chance for everyone to win.
Though I don't really know anything about balancing a game.
Of course, the more different fractions there are in a game, the lower the probability that your fraction will win. Since there is often more different fractions if there are more power roles, then yes, I guess that's true.