I will only play in leagues with 12 - 16 teams. With 10 teams or less, all the teams are too stacked for my liking, and there are sometimes players that are all-star worthy on waivers. I tend to find more than 16 teams to spread the talent out more than I would like. With 20 teams, somebody may actually have to use Chan Ho Park!
I'm just talking about mixed leagues here. When you start talking about AL-Only or NL-Only leagues, I don't know if it's even possible to go much above 12 teams. Not enough starters at every position to go around.
It's all mostly a matter of preference, but these are my reasons for sticking with 12-16 team leagues.