Large number of clients association degrade AP performance?

Question

Cisco recommends that no 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.
My questions is that if the client just assocate to the AP and not downloading any traffic, does the throughput still reduced? How much traffic (in term of Kb/Mb) does the client use to exchange to the AP?
I have some APs which have up to 150 users authenticate and associate, but little traffic were used. Should we add more APs?

Answer 1

If your AP has a Gig connection then it's probably 802.11N capable, in which case you're looking at 300 or 450Mbps at 5GHz plus 150Mbps at 2.4GHz.. that's why it has an interface faster than 100Mbps.
As for number of Users, bandwidth, etc.. consider two things.  In most cases, User traffic is bursty, so available bandwidth isn't as 'bad' as "802.11 Rate / # Users" (which is good news), but also consider that all L2 broadcast traffic and any multicast traffic is sent at the radio's lowest supported data rate (often 1Mbps), regardless of the speed your Clients think they're connected at.  As the number of Users increases, so does the amount of broadcast traffic they produce, and thus the load on the AP increases.
A loading of 25-30 APs per User is about right in an office environment, but that doesn't mean it's gospel.  In HD WiFi jobs in stadia we look at significantly higher figures than this, or in highly-demanding places where Users are known to be dealing with large files, a lower number is used...  There's no right or wrong, it's about balancing cost against functionality and against the number of RF channels we have available to us.