The maximum number of users that can be attached to one AP is normally limited by the processing capacity of the AP. Manufacturers will normally state the maximum number of users allowed in the product specifications. The number can range anywhere from 15 users for a small USB-style AP up to 256 users for enterprise class routers. Typical commercial routers usually can handle about 100 users.
To allow several users to simultaneously access the network, CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) occurs at the Physical and MAC layers ensuring that only one 802.11 user can transmit at a time. CSMA introduces more overhead causing data rates to be reduced further, as all users have to be aware of each others transmissions in order to avoid collisions. If there is a user that is out of range of other users, then CSMA does not work and collisions can occur ("Hidden Node Effect").
As more users access the AP, the overall data rate will be shared among all active users, and the data rate for each individual user will decrease. The number of active users can vary widely, from 2 or 3 for VoIP calls, to 20 or 30 for active web browsing. Some AP's have Quality of Service (QoS) features that can prioritize delay-sensitive traffic (ex. VoIP) according to the 802.11e standard.
