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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?

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  • If you really want to make perfectly clear that you're referring to the brother whose job is at the supermarket, and not to one of his other brothers, then you could write "His brother, the one who works at the supermarket, is a friend of mine." the one who, or in other circumstances, the one which, clearly indicates that a restriction is in effect. Relying upon punctuation to disambiguate the two meanings is not prudent, IMO.
    – TimR
    Mar 15, 2017 at 13:15

2 Answers 2

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His brother, who works at the supermarket, is a friend of mine.

This shows that he has at least one brother, who is a friend of yours; also, that this brother works at the supermarket. The commas separate the additional (and optional) detail about where he works.

His brother who works at the supermarket is a friend of mine.

This shows that he has multiple brothers, and specifies that we are talking about the one who works at the supermarket, indicating that this brother is a friend of yours.

If you don't know how many brothers, you could also say:

He has a brother who works at the supermarket, and who is a friend of mine.

In this case you state that he has one brother who works at the supermarket, without implying that there are or are not any other brothers.

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    Whereas it's quite true that without commas (i.e. - a defining relative clause) implies that he must have more than one brother, the converse (that a non-defining relative with commas implies he has only one brother) simply isn't true. Practically everyone has two grandfathers, for example, but the vast majority of written instances of his grandfather who was include a comma. The other grandfather may exist (he must have existed, at least) - he's just not pertinent to the context. Mar 15, 2017 at 15:44
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Most of what was commented is correct, but strictly speaking, the second sentence is incorrect. You shouldn't use a defining relative clause after a noun that has already been identified - in this case by the pronoun "his".

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