I really don't think the various campaign worlds should count. It doubles my numbers at least. If you did, you have to go back to 2nd edition, and end up counting most things twice, doubling numbers yet again. Frankly, CoC d20, Star Wars d20, and the rest are all stretching it, as far as different RPGs is concerned, but the mechanics are different enough to merit it I suppose. That's both the greatness and the failure of D20, it allows connectability, so star wars rules could be imported into FR or CoC, albeit with difficulty, but at the same time, who's coming up with REALLY new RPGs, with actual different mechanics? Imagine if we'd all been content to use the basic engine of Prince of Persia? You'd never have gotten the newfangled "Sands of Time" video game because all we'd do is make really cool new version of the old game, with spins and twists to be sure, but the industry needs new mechanics, like Decipher's CODA system or Alternity, or the old D6 star wars or Vampire and all the other White Wolf Products. Sure, they all have problems, but D20 couldn't have been made without some of them. And someday, when the oft-discussed 4th edition comes out, it will need to have original ideas from outside the D20 world.
so here are mine:
d20 (D&D, CoC, Star Wars, d20 modern, Arcana Unearthed (Ravenloft, Warcraft, OA, Urban Arcana, probably a few more like Ghostwalk))
CODA (just LotR)
Star Wars d6
Warhammer Fantasy Roleplaying (blegh, hopefully the new edition will work better)
Alternity (Star Drive, Dark*Matter, Gamma World)
2nd Edition (Planescape, Dark Sun, Ravenloft, Forgotten Realms, Spelljammer, Arcane Age, etc)
Basic Set? The old Rules Cycolopedia and several of the boxes that you could buy at Toys R Us in the early '90s (and that's really as far back as my nostalgia can go, unfortunately. I wish I'd been there for 1st edition)
And if Amazon ever cleans up their act, I'll have White Wolf's Vampire
so eight, if you count rules systems. A lot more otherwise