Gah, why am I obsessing about population dynamics when it's simply about the author wanting to subvert an elf trope.
Having 100 kids+ is a breeding strategy which in nature involves either a very high mortality rate (see: a lot of insects and reptiles) or a lot of infertile drones (see: bees, termites). You don't see that breeding strategy in a lot of higher order mammals as generally they invest a lot of resources into a smaller number of offspring with a comeasurably low mortality rate. See also the general drive to lower birth rate in humans as childhood mortality decreases.
The breeding rate mentioned of elves in this manga would lead to them swiftly outstripping the resources available to them eventually leading to a huge population crash. The exception would be a limited number of fertile couples with a lot of non breeders but that leads to issues with low genetic diversity and high chance of inbreeding. Issues around that have arisen in real world polygamous communities for example some offshoot Mormon communities in the US which have high prevalence of specific genetic diseases.