This is not working in despite of selecting category and sub category. If the author thinks I'm wrong, I'm ready to help him to test.

No longer works

option to sort by number of reviews does not appear after installing the add-on

This is working again! Thanks for fixing it. It once again works (on 1/15/2020 in FireFox 72) just as before.

After a search, you MUST select a department from the lefthand pane, or it won't work. Do that, though, and then you will have an additional option under the Sort dropdown listed as "Number of Reviews."

Edit: Stop fooling with this foolish, non-working app. Just append this to the end of your search in chrome or FireFox:
&sort=review-count-rank

Courtesy of stackexchange
webapps dot stackexchange /dot/ com /slash/ questions /slash/ 36828 /slash/ filter-amazon-by-products-surpassing-x-number-of-reviews

It used to work, kinda, now it does not work at all, and the Github link is dead. The developer doesn't care, he just erased it from his site and turn the feedback / email off.... Adios Sayadzad wasted an hour of my time.