Not sure I follow, but what I think you're asking is: is Ricky the only factory pilot of a CR250? i'm hardly and expert, but here's my rundown...
Answer: No. Fonseca is on a factory CR250, and LaRocco's bike might as well be a factory bike, even though he rides for his own team... supposedly gets factory parts.
Kawi only has one factory 250 guy: Lusk.
Yamaha has many: Reed, Vuillemin (sp??), Ferry (4-stroke), and probably others I forget off-hand.
Suzuki started out the season with three guys: Tortelli, Pastrana, and Roncada... but only Roncada is riding b/c the other two are hurt.
So really, Kawi is the one that started out being out-numbered there. The Pro-Circuit team is a Kawi support team, though I just read an article in another mag that says that their bikes are every bit as good as the factory stuff.
I imagine you see a lot of privateer Yamaha's b/c they're arguably the best bike to start with... consistently rated the top one or two bikes in most shoot-outs... either 2stroke or 4stroke. And no, I don't have a YZ, I have a KX... just reporting what I read
-James