For most schools - all but the most selective - ECs are just another way of illustrating who you are, what you care about, and what you have to offer the school community. It’s also a chance to develop skills that aren’t developed in the classroom and show that you have a life and an identify beyond being a student. At the very most selective schools - schools where everyone has top grades and test scores - it’s also one way to set yourself apart from the crowd. For that purpose, you would need to demonstrate an exceptional level of achievement in something. Doing lots of ECs just to fill in all the blanks on the common application (or to look ‘well-rounded’) is counter-productive. Everyone in that group of high achievers is fully capable of signing up for and juggling lots of ECs - many do. It’s not differentiating at all and it detracts from the effort you could have put into the things you really care about: sleep, friends, your real interests, etc…