What hurts your credit more, having several credit cards or cancelling all but one of them?

I went into the frickin Gap and they asked if I wanted a Gap card, which I stupidly umed was a membership type thing, only to find out after the fact that I had just opened another line of credit...I have perfect payment history and I only have one credit card, would it be better just to keep this line of credit open and not use it, or just cancel it, in terms of how it would affect my credit score?