In this world, there is a group of demigods that functions as a religious organization. This organization operates worldwide and defends the realm from threats of a demonic parallel realm. There are also places where the realms bleed into each other, allowing those demonic energies to corrupt certain areas and mutate the creatures there. This has led to the creation of various races that are hostile to humanity. Therefore, this group od demigods are necessary for humanity's survival.
This group functions out of monasteries, with one on every continent. They replenish their numbers by recruiting from that continent's population. There are about 1000 demigods in each monastery, give or take depending on the time period. Children around the age of puberty are tested for the "divine spark", a sign that they have been chosen by the gods to serve a higher purpose. This is a significant moment for a clan, and they are honored by society due to their contribution to protecting the realm. The children are then infused with the divine essence through a powerful ritual performed by their inductors that converts humans into gods. Those found compatible will be inducted with a 100% success rate, and raised at the monastery to become warriors. These children will lose their memories of former lives in order to devote themselves fully to the realms protection. The process also turns them sterile, to remove the ambitions of forming lineages. These demigods live between 1000 and 1200 years.
I want to make these special children a rarity in the world to make it a big deal when one is found, but I also want to keep the numbers of demigods stable around the figure I gave previously. This world is similar to ours, with approximately 7 billion people living on it. How can I make this work?