When deciding how to organize your photos among multiple libraries, one common thing to wonder is what size you should make each of your libraries. There is no single answer to this question, but here are some tips to guide you in making your decision.
The primary reason to keep a library from getting too large is performance. Larger libraries do tend to be slower when used in Photos or PowerPhotos, so keeping your library from ballooning too large can help keep day to day use responsive. The exact size will vary greatly depending on a number of factors, including how old your Mac is, how much memory it has, and what kind of disk the library is being stored on.
Is there a maximum library size?
No, there is no hard limit on the number of photos/videos you can add to a library. The library will tend to get slower as it gets larger, but you’ll never hit a point where Photos will prevent you from adding more items to the library.
Size vs. item count
When estimating how a library will slow down as it grows, the number of photos/videos in the library is much more important than the size of the library in GB. For example, a 900 GB library made up of 3000 videos will be much faster than a 900 GB library made up of 150,000 photos.
The speed of the disk that your library is stored on plays a major factor in Photos and PowerPhotos performance when working with a photo library. Generally speaking, your internal SSD is going to be the fastest, followed by external SSDs, then external “spinning platter” hard drives, and lastly network drives such as a NAS or another Mac accessed via file sharing. You might consider a setup where you keep your most recent and/or favorite photos in one library on your internal drive, and then copy older photos to one or more libraries on larger, slower external/network drives.
The age of your Mac, the processor it has, and the amount of RAM is has, all play a part in how fast everything on your system runs, including Photos. If your Mac is only a few years old, then upgrading to a whole new system probably isn’t worth the price. However, if your Mac is older than 5 years or so, and you’re experiencing slowness in Photos and/or your system as a whole, then getting a new machine might be worth it. This is generally viewed as a last resort.
Do a bit at a time
The best advice is to simply not worry about your setup too much ahead of time until you actually start to experience slowdowns. If you’re looking to merge a bunch of libraries together, try merging one library at a time, and then use Photos after each library gets merged in and see how it behaves. If it seems OK, go ahead and merge the next library. If it seems to be getting slow, then maybe you stop there and start a new library to hold all the rest of your photos instead.