Although there are other ways to do this, [Rod] has decided to write up his method of using two Arduinos and the I2C protocol ... be to buy an Arduino Mega with 54 digital IO pins or the Muxshield ...

The Uno also has a limited number of pins, making it difficult to work with when the project requires plenty of input and output components. However, these two areas are where the Arduino Mega shines.

I2C expanders, or ADCs will give you plenty of pins for whatever project you have in mind. All these require extra components, though. Enter the ExtraCore library for Arduino, a software library ...