Actually, the magic number is three:
A fire team consists of three marines, and one team leader.
A squad consists of three fire teams, and one squad leader.
A platoon consists of three squads, and one platoon leader (and a platoon sergeant and, I believe, a couple of other "HQ" guys)
Etc.
Of course this is purely paper strength. In reality, most units will be under strength due to insufficient recruitment, fatigue, sickness, casualties, and so forth. Particularly depleted squads may be merged with others, or padded with other USMC personnel (remember, "every marine a rifleman" means even truck drivers and cooks can be frontline jarheads in a pinch) leading to all sort of odd composite units.
Depending on the mission, fire teams and squads can also be modified, for example to fit into the transport vehicles made available, as these are usually not designed with any kind of unit coherency in mind.