When you install a new theme and the install works just fine but when you go to activate the theme it's not there? That is when you know you have too many themes. If you remove one and then all the sudden the new theme will not work is confirmation you have or had too many themes on your phone.
Each persons limit will be different because our memory use is different, I may be able to get more themes on my phone that someone else who had more apps on their phone.
Google Talk: srl7741