I guess you asked this question wondering the difference below:
1, I have many relatives, and I have never met any of them.
2, I have met some relatives, but I have not met many yet.
For the meaning #1,
I have many relatives, whom I have never met.
"whom" here = all of the many relatives
The antecedent of "whom" here is "many relatives".
For the meaning #2,
I have many relatives whom I have never met (while I have already met some others).
The antecedent of "whom" here is "relatives."
Another instance:
3, He has three sons who are doctors.
4, He has three sons, who are doctors.
Sentence #3 means he has more than three sons and that three of them are doctors.
Sentence #4 means he has only three sons and that all of them are doctors.
So, the antecedent of the 'who' in #3 is 'sons'; 'three sons' in #4.