Quick Question are you using a p/s2 (older connector, uses din 6 I believe) keyboard or usb?
USB keyboards are limited to a max of 6 keys plus 4 Modifiers. Modifiers are Shift, Crtl, Alt, Super key (aka the windows key) and ether AltGr or FN.
If you want n key rollover"aka unlimited button pushes" you will want a non-usb or ps/2 keyboard.
Also take note not all p/s2 keyboards have this feature.
As per your question, No the limitation to the number of max key presses in a keyboard is in the hardware keyboard encoder inside your keyboard. Personally I notice good keyboards are just as expensive as an arcade fight stick.