Just joined Lemmy today. From what I understand, there are different Lemmy instances that host different communities. I supposed that to interact in a community hosted in a certain instance, I would need to create an account for that instance. Ended up creating 10 different accounts.
Instances talk to all the other instances through federation. You only need one account on one instance- from there you can then go to communities on other instances by linking !community@instance.tld (e.g !memes@sopuli.xyz) and/or looking them up in the Communities tab and interact with those communities from your home instance.
We don't have a good sense of which instances are going to be durable long-term yet.
Additionally, If you only have one instamce, you're signing up for that server's bespoke curation of the Lemmyverse.
I currently have (I think) 5 accounts spread across Lemmy and Kbin for these reasons.