This question has come up a few times before and there is no one answer.
Personally, I like to try and target 25 users per AP, and that gives a pretty good experience.
I don't like to exceed 50 users per AP.
Some people have clients that barely use WiFi (perhaps low bandwidth scanning) and go up to 200 users per AP.
The more bandwidth you think your users will need, the less users you'll going to be able to support per AP and keep a good user experience.