I like to keep 2-3 games at a time that I'm currently playing, typically one single player and a couple of multi player titles. I never have more than 4-5 games at a time and I always trade them in when I'm done with them. The holiday season usually puts out a glut of games I want but since I can't get them all at once I get the multiplayer titles I want on release day to take advantage of the community, while the single player games get set aside for later months when the online dies down.
Currently I own 6 games, but 4 will be traded in shortly for new titles. I am also a little behind as I haven't picked up Brutal Legend yet. With Borderlands in my 360 now, and Dragon Age, Assassin's Creed II, Left 4 Dead 2, Mass Effect 2 and Splinter Cell Conviction all coming within the next 4 months, I have plenty to keep me busy well into next year.