If they all hang out on communities on the server (more or less), no problemo at all.
But if they all roam around and sign up on thousands of active communities on other servers, my server will be under water.
I love thinking about stuff like this (P=NP, complexity, etc) and I do not see very much about that concerning the lemmyverse which is IMO a shame.
I’m planning setting up a Lemmy build so I can tinker around with it, but you know, time and stuff. I also spent a lot of time just setting up the docker version so maybe it’s quite the job :-)