I actually posted something similar a couple hours after this, before seeing your post:
Well, they actually made several server-clusters for WoW Classic. So the question is more about adding servers to the full realms only, such that Bene-01 and Bene-02 would become a cluster, Grobb-01 and Grobb-02, etc.
They didn’t mention this type of scenario in the post that talked about technical limitations. They did make mention of layers or sharding not being possible solutions.
“One common suggestion we get is to “just add more layers”, and it’s very important to understand that layers do not add in any way to capacity.”
“This functions in some contrast to the “sharding” system that modern World of Warcraft uses which basically does the same thing but spins up additional shards on a per-zone or per-area basis.”