The Kirby Related Pages plugin provides a page method that fetches related pages based on the number of matches in a given field. You can set a minimum number of required matches. The result is sorted by number of matches.
$relatedPages = $page->related($options);
More details in the readme.
You can find the plugin on GitHub: https://github.com/texnixe/kirby-related-pages
I’m well aware of the fact that there is already the RelatedPages plugin by @Adspectus. This one works a bit differently and uses other options.