Probably more than 100 k in Silvermoon, even 200 k is possible (just in the capital, so a lot more around the Kingdom of Quel’thalas of course, and even more as some High Elves chose to live in Human Kingdoms like Dalaran or Lordaeron). You have to consider that during the Third War Silvermoon was a city that almost had 7 k years and the High Elves only suffered two major wars in thousands of years before the Scourge invasion anyway. Imagine Ancient Egyptians if they had survived as a culture until today, with only two or three wars in their history…they would have a very big population, more than Egypt has today for sure.
Indeed canonically the Blood Elves are now in the tens of thousands at the very least. We know 10% of the Blood Elves survivors were just the Sunfury of Kael in Outland , and the Sunfury were at least 2000 canonically (and I’m not counting other groups like the Scryers and the Blood Elves who remained under Illidan in this number). So both the Sunfury group and the High Elves who are 10% of the Blood Elves are in the thousands, Blood Elves in Quel’thalas are in the tens of thousands canonically.
Still realistically it’s quite difficult to rebuild and run a kingdom infested with Undead and Trolls for just tens of thousands of people, but as I said Blizzard is willing to ignore that because they had made a mistake with the death ratio in the Third War being way too high for the High Elves.
But this suspension of disbelief is present in other games, shows and franchises that have a zombie apocalypse in their story, actually, so it’s okay. For example in the Walking Dead show I don’t think the human survivors of the apocalypse are even in the thousands in the USA, but still they are able to form groups and small communities of people, and not just fight against the zombies but even against other groups of survivors, so…