The number refers to the number of items visible in the given view. If listed by title and your collection preferences, that means you’ll see the number of movies which are not part of a collection + the number of collections.
I have 455 Collections. Checking each one individually, is out of the question. So, I tried two different ways to check with a program.
I used Tautulli
I wrote a python script.
To verify that both methods can identify a single Collections with hidden items, I first purposely configured one Collections to hide its items. Both Tautulli and the Python script found the “Test” Collections. Then I set the “Test” Collections back to its default setting and no other Collections with hidden items were discovered.
Note: When I ran the test, based on what Collections was discovered, my “Title” sort count should have only been reduced by 10 but it was reduced by 12 instead. Which means my “Title” sort count is consistantly “-2” from what it should be.
I was able to synchronize all Movie titles across 5 different Plex servers. I used two servers and scrolled down the list of Movie titles, side-by-side, until I could see the shift in alignment.
Once I was able to discover which (2) Movie titles were missing, I was also able to search for them and see that they were also missing from Collections.
Once I forced the two missing titles to join the Collections, the missing 2 reappeared.