You will probably want to have more than one wireless network.
20 active PCs is too many for one wireless router or access point. If some PCs are used only rarely, then one connection would be OK.
You would need to have at least one access point to plug into the wireless router, and establish a second wireless network with it.
Alternately, use a wired router and connect two or three access points.
Multiple access points would have to be on separate non-overlapping radio channels, and the network names would have to be unique.
e.g.
wlan1 ---ssid "apple" channel 1
wlan2 ---ssid "baker" channel 6
wlan3 ---ssid "charlie" channel 11
the above channels apply to 802.11b wlan channels used in America, non-overlapping channels from other technology in other regions of the world may be different