You know, considering the MC has been fucking his mother and sisters…
With his three guard students and his sisters he could have a lot of daughters. If each of his wives (excepting the mom who will probably have only one kid with him) have three daughters that will give him 16 daughters. By the time they’re old enough to be bred he will just be hitting thirty. With his 16 daughters he could have another 48 children of whom one may be a son. Considering how dick crazed the women of this world seem I bet they would be able to maintain a high birth rate of 3 children per woman.
There is of course the issue of inbreeding depression, his hypothetical children of the daughter he had with his mom would have an inbreeding coefficient of 37.5%. They would share 87.5% of their DNA with their father. Unless there’s something special in this world’s Biology that makes inbreeding not an issue then this will be a problem.