Why our forefathers did not have more children:
Why did Avraham stop after a single child with Sarah?
Why did Itzhak stop after the twins?
Why (and how) did Yaakov stop after having the 12 tribes, where his wives were in their twenties?
I assume there was some divine guidance, like if G-d said 1 child or 12 children so it be, but I would like to have some sources that mention the reason(s).