If I were you I would make one big RPG. It's easier to get people invested and involved when a bunch of different stories can overlap, as opposed to "We're going to do this, and then we're going to do this, and then we're going to do this."
That way the players can always branch off and do their own things separate from the overarching narrative. One thing to remember, in anything really, is to not bite off more than you can chew, and to not assume things before you even get started. You might have enough for a whole series/universe of games, but people might only stick around to play one. Would you rather have one game that gets played, or two or three that die?
To each their own though.