I never understood why they put a cap on membership. Woudln't the families want to expand as much as possible like any business? As long as a guy meets a certain earning expectation (as well as some history of violence) that is a good enough to be made why wouldn't give him his button?
From what I understand, it was to keep the relative balance of power between the families. In other words, to prevent one family or another from trying to expand as much as possible and becoming a threat to the other families.
