How do you determine your game configuration?

[Moros]

How did you determine to have a good, balanced and fair game? How did you determine the number of mafiosi, doctors,...?

I'm starting up a game on another forum, where it's the first time this game is played. I currently have 16 entrees and hopefully we'll get a few more.

How many mafiosi would you think would be best, and what other roles could I add. I want to have a few uniques (depending on how many more poeple will join), but I don't want things to get complicated. As most players are new to this game. What do you guys think?

EDIT: make it 18 players.
Joining deadline moved to this evening. I'm not a good waiter. .