[@Arcarius]Same question I asked Renny.  Where did the Sith come from?  Dooku, Maul, and Sideous are the only known Sith.  If there are too many, then the Jedi would find out and wouldn't think they are "extinct for a millennium" as one of the Jedi Masters said.

I know it seems like I'm being a bit ridiculous, but if there are too many Sith, the plot of the movies kind of breaks down.  The Jedi would know that they exist, at least, and would be out hunting them.  Dark Jedi or members of other force traditions would be common enough for the Jedi to know about them.