Because honest to Ainu, somebody asked the question.
Male and female elves are born to male and female elvish parents. Elf mothers only give birth to true blood elves who will be immortal. That means that non-elf fathers and genetically diverse fetuses are non-starters. So maybe one in a thousand sexual encounters between elves leads to offspring. One in two fifty to five hundred when it’s the same set of parents for repeated pairings. But elves are not, by nature, monogamous.
Elf fathers can spawn Half-Elven offspring with humans, at maybe a quarter the pregnancy chance of human to human pregnancy. The usual human odds of babies not coming to term or infant mortality still apply, though.
Half-Elven parents have maybe half the conception chance of human to human, but are far less vulnerable to failed pregnancies and infant mortality, so it’s roughly a draw.