I strongly recommend not going over 15 schools. You should ideally have 3-5 reaches, 3-5 matches, and 3-5 safety schools. It is a lot of work that goes into each application, even with the Common App- each school generally has their own supplemental questions.
I think the people who apply to massive amounts of schools are either really nervous about not getting in anywhere, or want the “pride” thing of being able to say they got into loads of places. Either way, it makes your senior year much harder and makes you have to spend a lot of time in April making decisions.