I'm at 16 and I think around 12-14 it felt like I could rotate around the pins I'm playing and never get tired of any one game. I don't sell games a whole lot though. If you do, 6-7 maybe.
I'll tap out of space around 20 in the short term, so I'm being pretty selective now. I like the feeling of having a game I wouldn't mind selling in case something comes up I want and don't want to spend new money on a game. Right now, I don't really have one earmarked for that.
Long term I'm going to build a building for an arcade on some land I have.