Hi Amar,
This is a 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 would be 20-30 users per AP depending on actual applications etc.Have a read for some great answers provided in this forum by some "hands on" experts:
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=General&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.ee9a96d/2#selected_message
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=General&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd6c0a5/0#selected_message
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=Getting%20Started%20with%20Wireless&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd5f1ca/0#selected_message
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.eeab5db/1#selected_message
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.
From this Q&A doc;
http://www.cisco.com/en/US/products/hw/wireless/ps430/products_qanda_item09186a008009483e.shtml
Configuring and Enabling MAC Address Filters
MAC address filters allow or disallow the forwarding of unicast and multicast packets either sent from or addressed to specific MAC addresses. You can create a filter that passes traffic to all MAC addresses except those you specify, or you can create a filter that blocks traffic to all MAC addresses except those you specify. You can apply the filters you create to either or both the Ethernet and radio ports and to either or both incoming and outgoing packets.
Note: Using the CLI, you can configure up to 2,048 MAC addresses for filtering. Using the web-browser interface, however, you can configure only up to 43 MAC addresses for filtering.
From this doc;
http://www.cisco.com/en/US/docs/wireless/access_point/12.3_2_JA/configuration/guide/s32filt.html#wp1034897
Hope this helps!
Rob
