Messy extracurriculars?

Answered

I have the same situation as you, most of my extracurriculars happen outside of school. Thus, what I did is add some sort of proof like a facebook page link or a website. You can also add additional contact numbers in the "Add additional details" section in the common app.

Since you have more than 10 extracurriculars, I suggest narrowing it down to those you are most involved and accomplished in. Like how many hours you spent on an activity, or what activities demonstrates your leadership skills. You may also like to be mindful about the overall theme of your activities. It would help if you have a main theme regarding these. Admission counselors are more inclined to accept students whose interests are greatly presented and shows focus on a topic. Don't try to be an all rounder, since that may just look like resume padding. Instead, find your main interest. Is it science and arts? Sports and music? etc.

Anyway, I wish you all the best in your application!