And, of course, multiple personal names is a way of honouring friends as well as family.
There are a bunch of different customs about first names.
In France, at least for a while, first names had to be selected off a register of allowed ‘French-sounding’ names, to combat a perceived invasion of improper, unFrench names that were becoming popular.
In, I think, one of the Scandanavian countries, there is a tradition that everyone born on a particular day of the year has the same name, so someone told me about visiting wherever it was and suddenly people were saying Happy Birthday to him all day, six months from his actual birthday.
In the Catholic tradition, it is very very common to be named after a saint in at least one of your names, and for girls the saint is very very commonly Mary/Maria/Marie/Marion, to the point where priests have been known to add in Mary to the baby’s name when she was being christened if they felt that the mother hadn’t selected well enough. There are also customs in some places to celebrate the day holy to the saint whom you were named after (for instance, in Austria at the time Maria Von Trapp was growing up - she wrote about it in her autobiography.)