I don't have many systems (PS2, N64, Game Boy, GBA) but my brother has so many it's disturbing. Here's his list:
NES
Super Nintendo
N64
Gamecube
Playstation
Playstation 2
Sega Genesis
Sega Dreamcast
Game Boy
Game Boy Color
Game Boy Advance
NeoGeo Pocket
That's an awful lot, 12. I'm just curious if anyone can beat that. Well, not really, I'm mostly just bragging.