To clarify, what I'm asking is how many series would you use more than once or twice, beyond bit parts. To simplify, the series in question would likely be what you really like (and thus would want to use if possible/applicable) and what you're really familiar with. This question is posed to the fan-fic authors that exist, as well as hypothetically to those who don't.
My main three would be/is Ranma 1/2, Sailor Moon and Aa! Megami-sama. Others I would/will use often enough are Tenchi Muyo, To Love-Ru, Shugo Chara and Touhou. Only seven for me. But they're both what I'm familiar enough with and easy to use or crossover with.
(That's what I came up with after looking over the stories I've started, finished, and the one-hundred and twelve story ideas that may or may not be the next top model. Or something like that.)