In order to have a great credit score, it's important to have some credit cards in your wallet and a lot of available credit to use. So, will having too much plastic in your pocket hurt that solid score you have worked so hard to build?
The answer will always depend upon which "credit expert" you ask; however, what you should really be thinking about is your "credit utilization" ratio.
Run a quick analysis of how much you're spending of your available monthly credit limit from all your cards combined. This number should ideally be somewhere between 10-30% of your combined credit limit no matter how many cards you use. If so, lenders and credit bureaus will smile down upon you.
However, if you're running above this threshold, your credit scores are taking a hit since roughly 30% of your FICO score is determined by your credit utilization ratio.