How does having credit cards with no balance affect your credit?
That depends on a couple of factors. First of all, if you have too many credit cards then you might be considered overextended financially and your credit score would go down. If you are trying to improve your credit rating by just opening a credit card and not using it then you aren't helping at all. Having a credit card and never using it doesn't improve your credit because there is no record of how well you pay your bills if you never have a payment due. If you have an extra credit card or two and aren't using them they don't hurt your credit rating, and infact they might even be raising your credit rating. The extra credit card lowers your debt ratio since you have more avaliable credit compared to how much debt you have.
Having too many credit cards negatively affect your credit rating.
Also, you have many more numbers to loose or cards to steal from.