I don't think anyone would disagree with wanting unlimited heirs. As a practical matter, however, a limit probably had to be set for programming purposes.

I am not sure practicality is the main reason here. The Infant Mortality rate in the period covered by this game is very high. Up to 50% ( maybe higher ) of all births, could result in an early death. Six heirs, who all make it to maturity seems good to me. Based on the English Royal families of the period it certainly is!

IMHO anyone who wants / needs more than six is unlucky, foolhardy or maybe just plain greedy!