Enable the ‘Duplicates’ filter to see if several files have been misrecognized as the same movie.
Enable the ‘Collections’ view to see if collections have been created automatically and some movies have been ‘combined’ into a collection and thus removed from the total item count.
(This can happen with mp4/m4v files – even if Plex is not downloading collection info from TMDB. See the link about Collections.)
Some more tips about How to find the discrepancy between number of files on disk and the number of movies in the Plex movie library
