I think the key here is "in-depth." While it's great for a student to be able to fill up the extra-curricular section of the Common Application (which incidentally has space to hold up to 10 activities) quality is absolutely more important than quantity. Realistically a student might only have time for 2-3 activities they are heavily involved with along with a few additional short-term commitments. An in-depth activity should be defined as something a student is involved with for all four years of high school that requires a time and energy on a regular basis. Think of things like varsity sports, a long-term part-time job, or participation in a school band or drama club.
When I was working as a college admissions counselor it was much more important for me to see a student who was passionate and committed to their activities than to see a long list showing short-term periodic involvement. Consider also that a single student interest could spawn a list of multiple activities. For example, a soccer player might play for both their school and club teams, participate in fundraisers through a booster club, and volunteer time coaching a team for younger students. That would be a full schedule indeed!
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