We personally feel comfortable with only a few. Others can only handle one, and others yet can handle many more. There are countless factors affecting it all.

For us right now, it is one as GM, a second as player and a third as a prospective player.

The key factors we operate on is, imagination, workload/effort & time. For us, the RP we GM takes considerable work, probably about as much as two regular RPs. But it is also a slow-moving RP, with there being weeks between some posts and hours between others. 

We avoid joining more than three simply because we feel we can't take time to participate in more without at least one suffering.