I found the below examples in Cambridge dictionary under Defining or non-defining relative subject.
non-defining: His brother, who works at the supermarket, is a friend of mine.
He has only one brother, and that brother works at the supermarket.
His brother who works at the supermarket is a friend of mine.
He has more than one brother. The one I’m talking about works at the supermarket.
How do they determine if he has only one or more than one?
Let's say I don't know how many brothers he has, Which one should I use?