The mortal races are those which live normal lives, they are born, they live, they pay taxes (if applicable) then die. When a mortal being dies, their soul goes to Kinigi, where their soul is cleansed then reborn inside a new body.
While the number of mortal beings (and thus souls) is not infinite, there are a huge number of them. Mortal beings are often preyed on by Deitics, who lack souls and seek to enter the cycle of life and death themselves.
The strength of mortals comes from their determination to truly live their lives and not waste them in their short existence in the universe.