So I was re-reading the Dalaran books for the lols and I came across: "We've been attacked... Kael'thas' army... thousands of them!!"
Blizzard has already established population percentages:
90% of the HElves were wiped out in the 3rd War. Of the 10% that survived:
- 90% became BElves
- 85% of whom remained in Azeroth (7.65% of original population).
- Kael took 1.35% of the original population to Outland.
- HElves (1% of original population) were outside Quel'thalas during the attack or were later exiled.
Using the "thousands" figure (>~2k) for Kael's group of Outland BElves:
Original HElf population, >~150k
- Survivor population, >~15k
- BElves, >~13k
- Outland BElves, >~2k
- Azeroth BElves, >~11k (plus some returning Sunfury)
- Helves, >~1.5k
EDIT: '>' means greater than. '~' means loose approximation. This is only a ballpark approximation for the lower bound.