What percent cacao is semisweet chocolate

That's difficult to answer because different companies use different percentages. For example, ghiradelli uses 32% in their semisweet, but Scharffen berger uses 62%. If you are making chocolate cookies, use the one you like the most. If you are making a buttercream or ganache, I'd go with 40-60%.

This article may help you: http://food52.com/blog/9544-from-the-good-food-awards-how-to-choose-your-chocolate