Duane,
The only input pins on a parallel port are 10,11,12,13 and 15. The rest are outputs. That said you can configure Mach3 to set pins 2-9 on the 2nd parallel port to inputs as well, but that's not applicable if you are using just the one port.
So, you should be able to use any of these assuming that they are not already being used. On some setups the Mach3 Estop input is set to pin 10 or pin 15.
Have a look at Engineering note EN0098 on the following page. It shows how the limits are set up on my MB-02V6 breakout board and gives a table of the parallel port pin assignments.
http://www.homanndesigns.com/store/index.php?main_page=page&id=3Note that you can just daisy chain the limit switches rather than a separate input for each switch.
Cheers,
Peter.