They’ve talked about the full servers like Herod having a queue 10,000 deep. Realize that the original vanilla servers weren’t even close to that populated. I’ve seen 2500 and 3500 given in various places as server capacity but I’d imagine those numbers are meant to be concurrent logins, not total accounts/characters created on the server. I found this post https://i.imgur.com/yNWkujT.png from Mark Kern (Grummz) on the Elysium Project Discord says that the server capacity was 3500 to 4000 and I’m guessing he means concurrent logins. Additionally Blizzard ran the vanilla servers intentionally below capacity https://youtu.be/izCBaGfGJCg?t=2373 also per Mark Kern.
At some point during vanilla I believe they raised the cap to 3500 per realm. So if low means 3000 people then the low servers would end up being the only servers with an authentic vanilla experience which is why I’ll probably be rolling on one of the new servers Monday.