@Fazzahra5634 I guess there are two issuses here -
A) if we look at the number of potentional heir candidates, there are too many. Therefore, it is better to identify the "weeds" quickly and nip in the bud the potential, or Lack there of, of the bad/lazy/not Talented ones. If we look at their Career perspective, it makes better sense to choose between 2-3 prodigies and not 8 of them. They May later attack each other and destroy the household in the battle of the inheritance rights. If The 4-5 children know from the beginning they have no chances, then they can marry into another family, pursue arts, become a priest(an occupation a lot of 3rd and 4th sons pursue as a "safe" Option in middle ages)
B) it takes time and money to raise a Child into a proper heir. See this genius teacher in the manhua - he prefers to nurture the inquisitive ones, but if he had to take extra, i don't know, 4 h a Day to teach Bellesac ANYTHING, the others suffer from the Lack of attention. They are children still and need guidance. Now add apprenticeship, tours, piano lessons etc - Who has time and money to invest in a super risky investment?