Answer is as 'Gray' says, the easiest way to know is to try to put the words 'him' or 'he' into the sentence.
"Famous nappers include Winston Churchill, John F. Kennedy, Napoleon, Albert Einstein, and Thomas Edison, all of *who* are known to have valued their afternoon naps". = incorrect but used often in current English because many people think whom is archaic and should be abandoned.
You cannot replace the word 'who' in this sentence with 'he' or 'him' it does not make sense, so this is probably one of the last few remaining times where you can or should use "whom". even if it is considered archaic by many
1. Winston Churchill who was a famous napper.
2 John F Kennedy who was a famous napper.
3 Napoleon who was a famous napper.
4 Albert Einstein who was a famous napper.
5 Thomas Edison who was a famous napper.
Can all be replaced with "he"
1. Winston Churchill HE was a famous napper.
2 John F Kennedy HE was a famous napper.
3 Napoleon HE was a famous napper.
4 Albert Einstein HE was a famous napper.
5 Thomas Edison HE was a famous napper.
Ok to use 'who' when speaking of them individually plural has nothing to do with it, people will say 'who' in that sentence and it will only get debated in an English class but accepted by modern speakers in most cases.