How many credit cards is good for good credit?

[Xanthia Alysha]

I currently have 3 credit cards. A capital one, gap and just opened a citi bank a month and a half ago. I currently have a score at 684 and have had my capital one open for 2 years now. I am 21 but I want to make my score higher. I pay the cards off in full all the time but is one more card too many? will that affect my score if I have 4 or 5 cards?