First of all, it's fantastic that you've taken the initiative to start investing at a young age. One day you'll be glad you did!

There is no "right number" of mutual funds, but if you already have a few and you're looking to invest some additional money, I'd recommend diversifying your assets. I'm going to assume you have already maxed out your IRA's annual contribution, but if not, that should be your first priority. Beyond this, you may want to experiment with CDs or bonds if you're looking for stability, or you could play the market itself if you do your research. If you want to play the market, but are looking for a "safe" investment, there are plenty of companies with large market caps that aren't likely to shift radically.

Good luck!