No. You can have as many as you want. The limit is the resistor values. But the more you add the harder it will be to implement button combos. When you press more then one button you are putting the 2 resistors in parallel. If that resistance becomes the same as a resistor on another button you run into a problem. Which is what i have ran into trying to do 3 and 4 button combos. Because there is a 1 to 3 point variation in the values the analog pin picks up its easy for the values to overlap if the resistors, be it single or a pair in parallel, values are close.
EDIT:
you could always make a Graphic LCD version. The a joystick with a couple of buttons would be great. Add sound and you have a nice little gaming platform.
I will likely be doing a total rearrangement of what resistors are used.