There is one main essay for the Common Application and possibly other supplemental shorter essays you may have to prepare depending on the college's requirements.
Start early. Think of it as a creative writing assignment. Think about how you can show, not tell, a narrative about you that is not told elsewhere in your application. Write a few drafts. Have a trusted family member, friend, or teacher read it to provide feedback. There are a lot of helpful hints online. Here are some resources I like:
https://www.youtube.com/watch?v=HjcyS3-9gFI
https://www.collegeessayguy.com/supplemental-essays