It really depends on what categories you spend on, if you like to travel and want to learn how to use miles for flights in an effective matter, and how many cards you are willing to have.
I have 14 credit cards, but I only carry around 3, sometimes 4, in my wallet at any given point in time. But all 14 credit cards have their purpose.
What I believe to be the best two cash back credit card combo for most people:
Citi Double Cash - as mentioned, earns 2% on everything
Sallie Mae MC - 5% on first $250/$250/$750 on gas/groceries/bookstores (including Amazon) per month