Here is some info that relates to your two questions;
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.
You can decrease the radio coverage area of an AP when you modify the transmitter power level parameter. Have a look;
The transmitter power (mW) setting determines the power level of the radio transmitter. The default power setting is the highest transmit power allowed in a regulatory domain. Government regulations define the highest power level for radio devices.
Caution: The transmitter power level setting must conform to the established standards of the country in which the setting is used.
Generally, the transmitted power is reduced to limit the effect of RF interference. The reduction has a negative effect on the radio coverage. The transmitted power is directly proportional to the radio coverage area. Therefore, the weaker the transmitted power, the smaller is the radio coverage area.
If you perform a proper site survey, and remove possible sources of RF interference, you can use the lowest possible transmitted power value to decrease the radio coverage area.
This CLI command under the radio interface changes the transmitted power level to the maximum on an AP:
Access Point (config)# interface dot11radio 0
Access Point (config-if)# power local maximum Use this command to set the power level to maximum. Then, check how much throughput you have, and move the power level to a lower value until you reach a high throughput rate that stays consistent. You can also start from the lowest possible power level and increase the level until you reach a consistent throughput. This is because in some cases, if you do not boost the signal to the maximum level, the throughput and signal strength can change continuously and not remain consistent.