The correct answer is: it depends.
Usually, 20 Mhz channel can support 30 to 40 active devices for average use, such as web browsing and checking email. However, this number can vary greatly based on the capabilities of the client’s devices (such as 1x1:1 or 2x2:2) and applications that are being used.
Based on the device capabilities and Application you can determine the airtime consumption of the device. Once you have airtime consumption you can determine the active device the AP can support.
An 802.11 Access point is considered to be fully burdened at about 80 percent airtime utilization.
To estimate the number of devices supported on a single AP radio, divide the individual airtime required per device into 80 percent.
80/ single device airtime consumption = device per AP radio.
For reference, a free predictive wlan capacity planning spreadsheet is available for download at www.revolutionwifi.net.