Relative pronouns_which (rare usage)

1. Quantifiers and numbers with relative pronouns

 We often use quantifiers and numbers with relative pronouns:

many of whom - most of whom - one of which - none of whom
some of which - lots of whom - two of which - etc.

We can use them as subject, object or object of a preposition.

She has three brothers two of whom are in the army.
I read three books last week, one of which I really enjoyed.
There were some good programmes on the radio, none of which I listened to.

Source: https://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/verb-patterns/relative-clauses

2. Using  "which" to give more information

We often use the relative pronoun which to say something about a clause:

He was usually late, which always annoyed his father.
We’ve missed our train, which means we may be late.

Source: https://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/verb-patterns/relative-clauses

3.  Punctuation

IDENTIFYING CLAUSE
A clause that identifies the noun before it (tells you which ones) is  an identifying clause. No commas are used.
The kids  all of whom go to my school won the biking race.
The biking trophy was given to the kids (all of whom) you just met.

NONIDENTIFYING CLAUSE
A clause that adds extra, nonidentifying information is set off with comma(s).  The pronoun in the quantity phrase cannot be omitted.
The Jaguars, all of whom go to my school, won the biking race.
The biking trophy was given to the Jaguars, all of whom you just met. http://www.grammar-quizzes.com/clauses-7.html