For the purpose of writing my response to the "how many monster books do you own?" poll I went to my RPG shelf and counted the RPGs. Eighteen! Not counting different edition os Vampire, ForeSight. and Hero System as different games. And I am by no means a collector.
2-3: D&D (3 editions), Vampire (2 editions), and GURPS. I guess if I stretched it I could get seven: AD&D, D&D 3, D&D 3.5, Vampire, Vampire revised, Kindred of the East, and GURPS.
I voted 2-3: D&D and the old ICE MERP. If you subdivide things that use a d20, however, I'll raise the stakes to D&D, Stargate, Babylon 5, Mutants and Masterminds, and Silver Age Sentinels d20, bringing my score to 6.
Each line represents what I consider to be a game system. I guess an argument could be made to count AD&D and Hackmaster as one, but I do not.
Original/Basic/Expert D&D
AD&D 1E
Hackmaster
D&D 3/3.5E
ICE MERP
Kobolds Ate My Baby
Tunnels and Trolls
Star Frontiers
Gamma World 1E/2E
Gamma World 3E
Gamma World 4E
Lord of the Rings Adventure Game
GURPS
Conan (d20)
Redline (d20)
Spellslinger (d20)
Looks like it's time to put some stuff up on ebay.
I probably had something like dozens at one time. However, most of them were never played, some of them were poor purchases and I ended hating them after reading. A couple years ago I started giving away things I wouldn't play anymore. Right now I have only three, although one is at risk, as I kept it only because I like the art in it.