Check out http://www.myfico.com/CreditEducatio...YourScore.aspx

This web site is by the company that created the credit score (Fair Isaac). They've never released the exact formula for how they come up with the number, but they do provide some detailed information. It looks like the number of new cards is a small factor.

Some of the major credit reporting agencies, and maybe myfico.com, will let you "play" with your credit score after you buy it from them. So you can adjust the number of new cards you have and see what happens to the score.