Well, what is each meant to represent? Should communities be constructed around the concept of topics, or should there be a server for each topic?
And please don't use any variant of "everybody can do it whichever they want", because this just avoids the responsibility of offering a personal answer and shifts it to them.
Personally I think the first (communities=topics)., while servers should provide voluntary redundancy for each other in case one of the servers has an inconvenient change of policies or circumstances for the users.
But I am not on the creative team of Lemmy, so my vision might differ from theirs. Also, I'm willing to change my belief if more solid arguments are presented.
And when you get there, you're overwhelmed with options: