You have a total of 16 digital inputs on the OI. 4 are availible on joystick 1, 8 availible on joystick 2 (the same 4 from joystick 1 and 4 new ones). 4 are availible on joystick 3 and 8 are availible on joystick 4 (the same 4 from joystick 3 and 4 new ones).
If you are building a custom button box, you could always use 2 cables. One year, we even ran the two joysticks into our button box, and had 4 output cables, so we could get more buttons.
Like Joey said, you could also use a resistor ladder to hook multiple digital switches to an analog input.
Make sure to read the OI reference guide:
http://www.ifirobotics.com/docs/oi-r...de-2-21-05.pdf