I think they used to wax the cardboard. Maybe they still do?

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.