The more thralls are placed at your base, the more enemies will spawn at a purge.
The number of thralls you have in total is not relevant, only the number at your base.
The rewards do not seem to change.
I do not know if thralls that follow you or are in the base, but “returning home” do count, but I guess not.
It can still be useful to defend your base with a whole army - maybe the increased spawn numbers can be defeated with a large army. Haven’t tested that yet.
What is NOT advisable, though, is to start a purge at a base where you placed 40+ named thralls you carefully collected and curated over the years, but don’t want to risk in a purge, then place another 50 disposable fighters and archers on your walls for them to look cool.
Speaking from experience here. Seen the movie 300, when the army of 1000 nations descends on the Spartans? That’s what happened to my base. Its wall held fast, but the game didn’t. 