They had two, Jan was the younger.
Click link below for a catalogue! It lists 36 paintings.I presume that is your question, and that you do not want the number of PEOPLE named Vermeer.
Personally, I don't think their is a "magic number" when you suddenly have too many children. It all depends on the family, community and the amount of money that is available for the parents' use. I...
5,000,000
She has three kids and they all have a in them