Hi Lorenzo,
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.
Because this is a "Shared Medium" and the actual throughput for the AP is about 22-Mbps these 24 users will end up with a reasonable connection depending on what they are trying to run.
Have a look at this good Capacity Planning doc;
http://www.cisco.com/en/US/products/hw/wireless/ps430/products_white_paper09186a00801d61a3.shtml
From this Q&A doc;
http://www.cisco.com/en/US/products/hw/wireless/ps430/products_qanda_item09186a008009483e.shtml
Hope this helps!
Rob