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.
As @PhilipDAth said there is no one answer.
It will very much depend on what the users are doing when they are connected. Here at the school district I work in we aim for 30 at a time and that seems to work very well. The other day during state testing we had 50+ connected on one AP with no issues during testing.
The biggest variable when determining WiFI need is what kind of bandwidth do you need to support for each device.
To get an estimated number you'll need to get some information such as how many devices, their capabilities, and what applications they'll be using.