You can enter up to 15 experiences but you only want to include significant experiences. Do not include information just to fill up all 15 spots. Medical schools prefer quality than quantity.
In the Works/Activities section, you may also identify up to 3 most meaningful experiences out of the 15 possible experiences. When you designate an experience as a meaningful, you will be given an additional 1325 characters to explain why. In this section, include how the activity helped you grow, what impact you made, and why it was important to you.