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’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”.