What is significant about the number twelve?

Odd coincidences of twelves keep occurring, such as the fact they discover they are both turning twelve on December twelfth. They begin to wonder if there is some sort of connection between Vermeer and the number twelve. They stop on a stairwell, on the twelfth step and tap on the twelfth rectangle on the wall. They continue to tap along the wall until one of the panels finally gives. There is the Lady.