You should search in the internet for projects using AT keyboards with microcontrollers. I have seen articles that describe how to wire a certain number of dedicated switches to standard AT keyboard controller in such a way that holding one down does not inhibit the press of another to be able to send its code to the PC. This scheme I think can support at least a dozen switches on a typical kwyboard electronics module.

To do this you simply rip the electronics out of a used (or new but cheap) keyboard. Then wire your switches into the place the flex circuit normally connected to the electronics module.
Michael Karas