From the rules:
Quote Originally Posted by Beefy187 View Post
Set ups
Mafia: Starts with two. Wins if they kill Matsudaira Naritsugu.
Town: Kill both starting mafia. Survive 15 night phases. keep Matsudaira Naritsugu safe.
"Starts with two" and "Kill both starting mafia", suggests that it's possible for the mafia to grow, otherwise why use "start/ing" at all?