Prior to my pos abovet, I had restored the system photo library from a recent TimeMC backup, and the missing photos are still missing. I also had copied this recent backup copy into another directory on the MBP, so creating the second Photo Library on my MBP. Then using the method suggested by Leonie, I opened the libraries alternately as suggested, and this did not resolve the missing photos issue.
I note that many of my photos had been filed into Albums, and the album headings remain, showing either zero photos in the album, or one or a few photos remaining in a few of the albums.
I am not aware of any actions I've taken to cause this missing photos problem. There are posts elsewhere on the web about the Mac Photos app causing many photos to go missing when updating macOS for some of the recent release updates - currently I am running Monterey v12.3.1.
I must admit I do not check thru all my apps on the Mac after every macOS update after the update says it has completed normally, so based on user posts elsewhere on the web, it is possible the photos were lost during any one of my recent macOS updates. Short of restoring a number of older TimeMc backups and then manually trying to find all the 100's of missing photos, I've not seen any other way of recovering from this.
Regrettably, this missing photos problem has occurred for me once previously, about 6-12 months ago. At that time from a few web searches I found a solution which involved a few simple actions and then opening the Photos app whilst holding down a combination of keys (similar to Leonie's suggestion). Magically it worked. I assumed this error would never happen again for me, and if it did, I would again be able to find the fixit advice on the web - I now cannot again find this advice..
This is also a serious integrity problem for the Photos software which has been known for some time, and Apple still has chosen to not fix it.
