How to find out the rating of sites (by attendance) of a certain topic?

Question

Ratings

How can I make a rating on the attendance of sites of a certain subject?
For example, with the theme "means for weight loss." I can’t think of anything other than how to take the search results with such a request as a basis. But it is unlikely that the result of the issuance can be considered an objective rating.
Thanks in advance for any help.