What's a good number of credit cards or charge cards to own?

Each individual's case is different, so there is no one correct response. But there is certainly a way to know if you have too many! (source)

For example, if you're walking around with cards you haven't used for a long time or, even worse, completely forgotten about – it's time to do something about this. It doesn't necessarily hurt your credit score, but with each credit card you own, it opens you up one more time to the potential of identity theft.