Remember when forums would be super active with, like, 500 users?
"Millions of users" is a vanity stat. The critical mass needed to keep a discussion group alive is actually quite small -- assuming you're interested in, you know, discussing things. So, how active "Lemmy" is is entirely dependent on which topics you're interested in.
No. Federation means I'm on a mbin serner and still interacting with lemmy. If a community goes big there is no way to enforce who goes to which split.