..., most of whom ... (formal relative clauses) (B2 Grammar)
To say how many out of a group in a formal way, join the two halves with a comma and of whom (people) or of which (things). Use words like all, some, most, none, both before of.
all of whom, some of which
She has three sons. They are all doctors. → She has three sons, all of whom are doctors.
I visited five countries. Most were in Asia. → I visited five countries, most of which were in Asia.
They invited twenty guests. I knew some. → They invited twenty guests, some of whom I knew.
Not "She has two sons, both of them are engineers." — say "She has two sons, both of whom are engineers.". Inside one sentence, use of whom for people, not "of them".
Try it: We interviewed ten candidates. None of them were suitable. Join it into one formal sentence about the candidates. (Leave out the word you don't need.) Answer: We interviewed ten candidates, none of whom were suitable. — To join it into one sentence about the people, use of whom: none of whom were suitable. "None of them" would start a separate sentence, not join this one, so leave it out. Inside one relative clause, people take of whom. (After none, both was and were are heard; were is the safer everyday choice.)
the house in which I grew up
In formal English, a small word like in, to, on can come before which / whom. It's the formal version of putting that small word at the end.
The house I grew up in was sold. → The house in which I grew up was sold.
The person I spoke to was kind. → The person to whom I spoke was kind.
The idea it is based on is simple. → The idea on which it is based is simple.
Try it: The company I used to work for has closed. Make the formal version, with the small word before the relative pronoun. (Leave out the word you don't need.) Answer: The company for which I used to work has closed. — When the small word comes first, it pairs with which, not that: the company for which I used to work. "For that" is not used in formal English; after a small word like for, use which. So leave "for that" out.
, which (a comment on the whole idea)
To comment on the whole idea in the first part (not just one noun), join it with a comma + which. Here which stands for the entire situation you just described — use which for this, never that.
He arrived an hour late. This annoyed everyone. → He arrived an hour late, which annoyed everyone.
She passed every exam. This surprised no one. → She passed every exam, which surprised no one.
They canceled the trip. This meant we lost our deposit. → They canceled the trip, which meant we lost our deposit.
Not "He arrived an hour late that annoyed everyone." — say "He arrived an hour late, which annoyed everyone.". To comment on the whole idea, use comma + which, never "that".
Try it: He arrived an hour late. This annoyed everyone. Add a comment about the whole thing that happened, using a comma and which. (Leave out the word you don't need.) Answer: He arrived an hour late, which annoyed everyone. — To comment on the whole situation (the fact that he was late), use a comma and which: ..., which annoyed everyone. "That" can't refer back to a whole idea this way, so leave it out. For a comment on the whole clause, it's always comma + which.