It depends on your needs and your spending habits. For some people, not having a single credit card is the wisest thing to do. However, if you have a need for credit cards, one should suffice. You do not really need more than one as you will find it easier to fall into the credit card debt trap if you have two or more credit cards.
However, if you are quite responsible when it comes to your finances, you will do well with two or three credit cards. Sometimes, having several credit cards can help you boost your credit scores.