So on the Common App, you'll fill out all your details, etc and your extracurricular activities, honours, etc. You'll also write one essay (minimum 250 words, maximum 650) from a list of prompts. If you google "Common App 2018-2019 essay prompts" you'll find them. There are loads of websites online to help you write your common app essay (I also recommend /r/ApplyingToCollege on reddit, super helpful!).
In the common app, you'll add your colleges. Some colleges require supplemental essays. For example, Vanderbilt asked to expand on an extracurricular activity, and that was it. Whereas some colleges may require a few essays (most of the time they're about 300 words max, some go to 100 words max). Some might even be really simple and not even be an essay, for example Wake Forest asked me my top 5 favourite books.
So to summarise: you'll write one 650 word essay on the common app, and the supplements for each college. The amount of supplemental essays/questions (or lack of), depend on what colleges you apply to. Everything is through the common app (apart from financial aid stuff).