They are a separate race. They have distinct traits shared among the population that is not found in Seekers, and the note of them being rare is presented as follows:
It's referring specifically to Keeper of the Moon males. I took it as meaning that Keeper males were rare and that Seeker males had a higher birth rate - hence the competition among them for the status of Nuhn.
Quote:...rarely do even the largest Keeper of the Moon families have more than two or three sons. This is not by choice. Nature merely sees to it that more females are born to this race.
It's referring specifically to Keeper of the Moon males. I took it as meaning that Keeper males were rare and that Seeker males had a higher birth rate - hence the competition among them for the status of Nuhn.