How to add more inputs on Arduino?
There is a whole concept you need to learn:

Matrix

By using a matrix arrangement you can, using (8 + 9 = ) 17 IO pins you can create a grid of 8 × 9 buttons, which is 72 buttons.  

This method is used all the time for keypads, and you can read more here: http://playground.arduino.cc/Main/KeypadTutorial

To further reduce the number of IO pins used you can replace the 8 IO pins for one dimension of the matrix with a 3-to-8 decoder, since only one pin is ever active at a time.  That reduces it down to just 12 IO pins for the whole shebang.