I only buy games that I want to play.
I'm at 20 games or so. I basically only own PS3 exclusives since I buy multiplatform titles on the 360 due to the controller and Xbox Live.
The only exceptions are Dirt (I enjoyed the 360 game so much I wanted to also see how the PS3 port turned out), IL-2: Sturmovik: Birds of Prey (The PS3 version was superior so I passed on the 360 release), and Sonic's Ultimate Genesis Collection (Also have it for the 360 but wanted to see how it turned out on this platform).
I'd be very surprised to ever pass 50 retail titles for this platform.