They would have to be to support five or six layers. But once realms are down to a single layer it's a little bit of a leap to assume that the caps are still the same. It's just as likely that as layers were removed so the server cap was reduced. You don't KNOW that they weren't just as I don't know that they were. But the evidence still suggests that the Blizzard eventual goal for realms is around the 3,000 level.
All of this is entirely speculative. No one but Blizzard really knows how it's working at this point. Just that layers are gone. Oh, and the game somehow survived their presence despite about a million words to the effect that it wouldn't.