Just in case you wondered how many photos you really can have in your Apple Photos library, I can report that I have so far added 1 000 264 Photos and 10 242 Videos without any issues.
I’m fairly impressed and happy about it since all I could find was that it should support up to 100.000 photos, with a few reasoning about the limit being increased to 300.000 on modern hardware.
I’m running this on my MacBook Pro M2 Max with 64 GB ram.
Most formats gets converted to HEIC and HEVC on import, which are staggeringly effective compared to their original formats. The whole library file still only takes up 1.7TB, which is much less than expected. The original source on my NAS is around 5.6 TB.
Edit: Maybe I should add that I do not recommend this, and view it as an experiment for now. I’m still importing data. If it’s still stable and performant after a year and some OS updates then I can start recommending it.
If Apple chose to use a 16-bit integer for the index, they would be limited to approximately 65,000 photos. 32-bit would be about 4 billion. Photos likely uses a 32-bit or even 64-bit integer index value to make it a non-issue.
There could be issues with having too many thumbnails loaded in memory. Maybe some other value overflows when scrolling through (or just loading) an enormous library. If Photos also creates an index for searching that could also create headaches. If Photos were a simple file explorer having a million files shouldn't be an issue but it's more than that, so it's good to know it seems to be performant with even 1 million photos.
They use a 64 bit integer for their primary keys.
https://github.com/RhetTbull/osxphotos/wiki/Photos-9-(Sonoma)-schema