Here's why:
* One entry for herself: Every teenager between the ages of 12 and 18 is automatically entered into the reaping.
* One entry for Prim: Katniss volunteered as tribute in her younger sister Prim's place when Prim's name was drawn in the reaping. This meant Prim's name was put back into the bowl, giving Katniss an extra entry for the next year.
It's important to note that Katniss only had the extra entry because she volunteered for Prim. If she hadn't, Prim would have remained in the reaping pool, and Katniss would have had just one entry in her 16th year.