Can I show the amount of search results when using exp:low_search:popular? And I would like to sort by the amount of search results if that is possible.
You mean the actual amount of search results if you were to search your site with those keywords at that moment? If so, no, that's not possible. Performance would take a nosedive (as you're performing X searches in one go, where X is the amount of popular keywords) and you'd have to take other filters into account too, which could differ in countless ways.