One thing about all these manhwas that involve succession of the throne is that there always more than 4 kids da** U telling U opened Ur legs to give birth to so many kids or that U slept with so many women to give U that many kids 
Comment by Chichi Louisa on Swordmaster’s Youngest Son - Chapter 5
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@Chichi Louisa I mean they marry multiple high-status women to increase the chances of having a highly capable heir.