How to use the Pigeonhole Principle to prove a DFA has a minimum number of states?
http://www.owenstephens.co.uk/blog/2014/09/28/NFA_DFA.html

https://cseweb.ucsd.edu/~ccalabro/essays/fsa.pdf

Evil's link and this other link seem to show how to apply the Pigeonhole Principle to show how to answer my problem. The second one explains it more visually and in a more noob friendly fashion.