Unless one gets a large van, daily life becomes less convenient beyond 2 kids.
Midsize sedan/SUV aren’t great at more than 2 parents + 2 kids (especially with car seats!)
Vacations are also more expensive since a lot of hotel rooms don’t allow more than 2 adults + 2 kids.
More is also harder on the parents !
Hotel + plane flights is too expensive for large families.
I try to drive and stay in Airbnbs, and then doesn't matter how many kids.