To estimate number of users, start by considering application throughput requirements for your WLAN users. Divide your AP's aggregate throughput by bandwidth per user to determine the average number of concurrent users. Throughput varies by AP and with distance, so you should really take sample throughput measurements in your own WLAN to estimate aggregate throughput in the coverage area where users will be located.
However, if you're planning your WLAN and you have nothing to measure yet, you can start with a rough estimate that peak throughput will be about half the maximum link speed, for users fairly close to the AP. For example, if you're installing 802.11g APs, you might get 20-30 Mbps aggregate throughput for nearby users connected at 54 Mbps. If your average user requires one Mbps, you might then estimate 20 active users per AP. No matter what you estimate, you'll want to verify actual results after AP installation and adjust your WLAN layout to meet user needs.