Elven survival is an oft argued topic.

One answer that I quite like is that 'the weakness is all human mis-perception'. The elves suffered terribly in the war of the shadows and Deismaar, but ever since have been recovering. As however the elves are not naturally expansionistic, and few humans ever enter their realms, the growing numbers are simply unknown to the outside world.

Under this sort of approach the Hanner Sidhe are popular, as a campaign story arc is then the elven resurgence which tends to require enough elves to hold much larger lands.

Other points are the shadow world : Cerilia split. With the Shadow World still containing spirit world areas these may be elven (or at least Seelie refuges), other elves could simply not answer a crown, or be experiencing an unusual passage of time. If you want to beef 'hidden' numbers then as a fan of morphic elves I can see many living the seas, deserts, polar regions, etc.

For me much of the problem is simulationism, elves are eternally young immortals with minimal industry that are immune to diesease - the only thing that really can kill of large numbers of them is a war, and the elves are not in the main a profitable opponent. Their numbers should therefore be steadily growing since Deismaar - particularly when the warding realm spell is taken into account! Yet if the population now is 8 times the number post Deismaar, the elves would have had to die in insanely high numbers in the battles (8* = 3 doublings = 1 doubling every 500 years - 1 child every century with 1 lost before they mate).