For home use, I use Kodi to watch media content. I provide Plex for a couple of others who provide content. Kodi reports 1324 movies, whereas Plex reports 1322. I logged onto my NAS and ran a count to confirm the actual number and it comes up with 1324.
Plex doesn’t show a number of movies but visible items. This can be off if your libraries use collections and are configured to show both collections and their items or hide items which are part of a collection.
→ sorting your library by anything but title won’t consider collections and should therefore match the number of movies in your library.
In addition, Plex might consider multiple files to be different-quality versions of the same movie. Different versions are grouped under the same movie item.
→ filter your library for Duplicates to identify movies with multiple versions / files.
If you follow a specific naming, you could also end up with stacked files. Plex is using this naming to append files which reflect multiple parts of the same movie (e.g. files with the same base name and a suffix like -pt1, -pt2…