This is a very good question that comes up every once in a while. As you can see from the answers in the links below,there are alot of factors that come into play. The consensus from everything I have read and experienced would be 20-25 users per AP depending on actual applications etc.
Have a read for some great answers provided in this forum by some excellent "hands on" experts:
Here is a Cisco doc that relates to this question;
Q. How many clients can associate to the AP?
A. The AP has the physical capacity to handle 2048 MAC addresses. However, because the AP is a shared medium and acts as a wireless hub, the performance of each user decreases as the number of users increases on an individual AP. Ideally, not more than 24 clients should associate with the AP because the throughput of the AP is reduced with each client that associates to the AP.