Please look up a decent network services shop in your area. It will be well worth the little extra money to have this set up correctly and securely for business use.
Also, any stationary machines should be wired. It is faster, more secure, more reliable, and will alleviate some of the headaches involved in getting that many wireless devices to play nicely together.
Even commercial devices will have a problem. It is not so much the router/ap as it is you are running out of radio bandwidth. You really should use 2 radio device if not 3. It really depends on what the users are doing. 2-3 even users running even moderate video streams can eat all the bandwidth.
You could just add 1 or 2 AP to your current configuration putting them on different channels. You would then have to use different SSID or physical placement to distribute the users between them.
As recommended above cable as many machines as possible. These machine will get dedicated path and it reduces the competition for the wireless bandwidth for the devices that can not run wired.