Well an account on a lemmy instance is an account on the fediverse, since Lemmy is a part of the fediverse. That all is to say that you should only need a single account on a single Lemmy instance (just pick one that you like the vibes of) and you’ll be good to go. For example I come come from lemmy.sdf.org but I’m able to post over here outside of that instance and subscribe to kbin communities.
Going back to the email analogy, it’s like how users on Gmail can chat with users on outlook and with users on other email providers without any issues - they all work together (mostly) seamlessly!