20 and more is getting real difficult for me. around 12 seems to be perfect. When I play less I can recall a lot more games even months later, at one point I had 30 and more going and I barely remember 3 of them.
I think a good way is putting the computer away for 5 minutes and then trying to recall the positions of your ongoing online games: If you can't even come up with half of them i think it is an overload of games.
With recalling I don't mean that you need to know the whole movelist or the exact positions, I mean that you remember some key ideas, something you remember the next time you see the position.
Have fun playing though, no matter if it is one game or a thousand
I agree that it depends on the individual. I've known a player to have 150 games and a 2000+ Online rating, all with a FIDE 2110 rating.