For the supplement essay, how many topics is required to pick

I'll edit my answer if needed once you respond to my comment but for now I'll try to give an answer that covers a few areas.

The number of supplemental essays you're going to need to write will depend on how many schools you apply to which require one. If you're applying to schools outside the top-20 you can probably expect to write 1-2 supplemental essays per school. Schools inside the top 20 typically require more essays but that's because they're more competitive. The Common App opens officially on August 1st so you can look up all the schools you're interested in and see how many require supplements. You can also check on here by going to https://www.collegevine.com/essays and typing in the schools you're interested in. The prompts you'll see are from the 2020-2021 application cycle.

If you're asking how many topics to include in a supplemental essay it will depend on the prompt but I don't see many reasons to include more than a single topic. You often don't have much space to write a response so you should focus on writing a clear, concise response to the prompt that's provided. Hope that helps!