What would be the recommended maximum number of pictures for use in PS19 "Organizer".

What is your concern about the number of photos?

I have never seen a recommendation by Adobe concerning the maximum number of media files in a catalog.  I currently have 150,000 photo and video files in my catalog and I have seen others report substantially more than that. 

I DO use face recognition and the other auto-analysis tools such as Auto-creations which is a new feature of 2019.  The size of the catalog does affect how long the analysis will take.  And while the analysis is in progress, the performance and stability of the program may be affected.

Your computer hardware will also be a factor in performance too.

stever31974073  wrote What would be the recommended maximum number of pictures for use in PS19 "Organizer".

Read this:

Can Elements 15 backups be restored into Elements 19?

I have 67 000 items for a library of 540 GB. I could easily have twice that  number of items. But note that I don't use face recognition.