Max size of Photo library

The optimal size of your Photos Library is a trade-off between the following aspects:

• The convenience of having all photos we need for some task together in the same Photos Library
• The speed of Photos when working with it or updating the library and scanning it again after a system update or upgrade. The large the library, the longer it will take to scan the library, repair it, rebuild it or sync it again with iCloud. If the library grows beyond 50000 items, these tasks can take several days, even weeks. When I ask at the Apple Store, what is recommend, I have been told, keep the library size below 20000 items.
• The structure of the library, particularly the number of smart albums. When I have created many smart albums, Photos will be slow, after importing new photos or deleting photos, because all smart albums need updating when photos are added or modified. With more than 200 smart albums Photos is slow as molasses for my library with currently 60000 photos and videos.
• The ease when navigating the albums or list of keywords, etc. I like to use keywords and albums, but Photos does not support long lists of keywords or albums well. There is just one list, difficult to scroll.
• The amount of storage on your device. If you are using iCloud Photos and your system library will be synced to your other devices, keep your system library small enough to work well on your device with the least storage. This will be the bottle neck.
• Our patience. Would it matter to you to have to wait a few seconds after deleting a photo, before you are seeing an effect? or a photo to appear in a smart album, after editing the metadata of the photo?

Theoretically, Photos should be able to handle a library with one million items. That has been the limit for Aperture, and Photos could migrate Aperture Libraries to Photos (until macOS 12).

The optimal size will depend on how you are using Photos and on your patience. Are you using iCloud Photos? Are you using many smart albums? Is your library on a slow external drive?

I am trying to keep the size of my system Photos Library below 50000 items. For larger libraries Photos is no longer responsive enough when I am working with it, even on my MacBook Air with M2 chip and 2TB of internal storage. This library is also fitting well onto my iPhone and iPad.

I am archiving all photos in smaller yearly libraries on an external drive, also separate libraries for each major event, like a photo safari. This way I can easily find certain photos in one place by opening the library for this event. My system photos library on my Mac is holding only a selection of photos, the photo I want to be able to see on all my devices at all times. It is just holding the photos that I am currently working on plus my favorite photos from all years, family photos and documents from all years, my best travel photos.

You're welcome, Marco_Polo. Good Luck 😊