For me, this is how I tend to do it:

Free: Up to ten, though really as many as it catches my interests. They never last long and I just join them for quick, novelty characters.

Casual: Roughly five, give or take. Note this only counts for Casual RP's that actually has some IC activity going on; if we're still in the OOC start up phase, I don't really count them.

Advance: One or two. I hardly ever join Advances as I'm more of a "High Casual" person, though sometimes I might know a guy who needs an extra body and likes what I can do.