1) they are the ones who remained in Middle-Earth, while others (e.g. Elrond's wife) departed west over the Sea to get away from the violence of Middle-Earth
2) they are ageless, and seem to have a very high adult:child ratio and essentially no elderly, so nearly the entire population is "military age"
Taken together, this could plausibly give not much under 50% able to serve in the military in extremis. Relative to Hobbits, Orcs, or Dwarves, the Elves are biologically far more different than Men.
1) they are the ones who remained in Middle-Earth, while others (e.g. Elrond's wife) departed west over the Sea to get away from the violence of Middle-Earth
2) they are ageless, and seem to have a very high adult:child ratio and essentially no elderly, so nearly the entire population is "military age"
Taken together, this could plausibly give not much under 50% able to serve in the military in extremis. Relative to Hobbits, Orcs, or Dwarves, the Elves are biologically far more different than Men.