But it’s just impossible for them to outnumber the high elves.
It’s a simple matter: using the last canonical numbers known, the WoW Encyclopedia, of the surviving elves of Quel’thalas, 10% rejected Kael’thas’ path and remained high elves. So there’s something close to one high elf per nine blood elves. And notice that’s the conservative number, since the exact words of the Encyclopedia can be interpreted in other ways (do they count the elves that were not in Quel’thalas, and thus weren’t among the survivors of the Scourge?).
Anyway, let’s take this 9-to-1 proportion and roll with it.
Another canonical number: Kael’thas’ forces were comprised of 15% of the blood elves. So, the entire armies you fought in TBC, ignoring defectors like the Scryers, were just a little more numerous than all remaining high elves.
FOr the void elves to have greater numbers than high elves, it would mean Umbric’s group had more than 11% of the blood elf population. His followers, mostly scholars, would need to be an army comparable to Kael’s.
I mean, if there’s 50,000 blood elves remaining (hypothetical number), then there would be 5,555 high elves living in Dalaran, Stormwind, Terokkar, Quel’danil… And you’d need to have 5,555 Umbric followers living in a barren rock in space (which would decrease the blood elf numbers by the same amount, BTW).
An army living in a rock in space, without food, water, farms or any supplying routes. There’s no shelter in Telogrus, just some tents. Do you think it’s feasible? Do you really think 11% of Silvermoon would leave to study dark magic?
In-game, the number of void elves currently in Telogrus number less than high elves in just Allerian Stronghold.
To consider that there would be more void elves than high elves is pure madness.