What does 1 item mean?

It could also be an audio file or an unknown image format.

Create a smart album with the rule "Photo is unable to upload to iCloud Photo Library". Some kinds of "Items" are incompatible with iCloud and will turn up in this album.

To narrow down the search quickly, you can also create a smart album with the rules:

Combine the rules with "Match All", not "Any". And use the most frequently used filename extensions for the image formats in your library.

It is something other than a photo or a video - maybe a PDF as one possibility

LN

I found some wav files in my Photos Library, that had been migrated from the Aperture library. I had used the wav file as the sound track for a slideshow.