what routher i need to connect 20 pc by wifi

Solution
For that many, you would want a router with several access points setup, 20 computers on one wireless connection would be pretty slow. You could get a Linksys E2100L for the main router, then get something like this http://www.bing.com/shopping/linksys-wireless-n-access-point-with-dual-band-wap610n-wireless/p/08DA5FC1BE85C45F0001?q=linksys+access+point&lpq=linksys%20access%20point&FORM=HURE for half the computers to connect to. You can setup different SSID names or use the same one for both devices.
For that many, you would want a router with several access points setup, 20 computers on one wireless connection would be pretty slow. You could get a Linksys E2100L for the main router, then get something like this http://www.bing.com/shopping/linksys-wireless-n-access-point-with-dual-band-wap610n-wireless/p/08DA5FC1BE85C45F0001?q=linksys+access+point&lpq=linksys%20access%20point&FORM=HURE for half the computers to connect to. You can setup different SSID names or use the same one for both devices.
 
Solution