Thank you Avinash for clarifying.
We don't have any such app or report available, but perhaps more importantly, I'm uncertain what the requirement you presented means:
Custom Fields are part of artifacts (part of defects, user stories, portfolio items etc .. ), so perhaps what you mean is finding out how many artifacts use these fields vs. how many are not. So, for example if you have 1000 defects, then how many have a certain custom field with a value that's set. I'm not sure how you can tie it to a user. Do you mean the owners of the artifact perhaps?
Either way, like said, there isn't such an app or report. There is this app called: My Custom Fields which shows the custom fields by artifact-type, but it doesn't seem to come close enough to what you asked.
I hope this helps.
Sagi