I made an alt on each of the big instances, because I wasn't sure which one I wanted to use long term. Then I just kinda stuck with them, each following slightly different topics, and therefore having different opportunities to participate in discussions.
Started out with lemmy.one, then when they had major downtime, I started on kbin, but when I couldn't use that with my preffered lemmy app, I spun up lemmy.world which is now my main.
Just one. Technically, it's this one. My "main" account is my kbin.social account, but it's so frequently having performance issues that I've been using this account almost exclusively for a while now.