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.
For the most part as highlighted above, yes only one account/instance and you can engage with anything that instance federates with.
However there are already cases where large instances have defederated from others, in which case you will not be able to engage if your local has been defederated from a community you want to access.
For this reason it is a good idea to have multiple, but use one as a primary.
I was on beehaw before and didn't like that losing access to lemmy.world so found lemmy.one. It's good to diversify.
I've also found it useful to have multiple to compartmentalize the subjects I subscribe to on each and limit what I see when I am in different "modes".
Makes it easier to see was is active in communities local to that specific instance too.