English grammar - WHO, WHOM, THAT, WHICH, WHERE ...
https://www.learn-english-today.com/.../grammar/who-whose-whom-which.htmlWhom is always used when it is preceded by quantifiers such as all of, both of, few of, many of, several of, etc. For example: She addressed the spectators, most of whom remained seated. The mayor welcomed the boys and girls, all of whom wore their school uniform. He greeted the soldiers, many of whom were injured.