Mac Photos app isn't showing all photos imported from my Mac

First, a caution: That choice to "Copy items to the Photos library" isn't there for any normal use. Photos is a database that connects pictures to edits, comments, dates, and everything that happens in Photos. The database depends on having full control of everything!. As you have it, moving an original or doing much of anything with the original files may totally disturb the database and make your Library largely unusable. We have seen this happen. Personally, I keep the originals in Finder Folders as backups on external drives, but they are not used by Photos.

Photos is a non-destructive editor. It keeps the originals inside its Library package, untouched. Except for thumbnails and temporary use, edited versions of the pictures are not maintained. Edits are simply memories in the database that are re-applied whenever an edited image is required. (The one change is that the filenames are renamed to unique identifiers--otherwise there might be many files with the name IMG_0234, or whatever. The original filenames are stored in the database, and most users never see the special Photos names.)

So, if I were you, I'd start over again with "Copy" checked. Another option, especially if you've already added information to the pictures that you don't want to lose, is to choose File>Consolidate to bring copies of the pictures into the Library and associate them with information in the database.

How many files are you trying to import at a time? Are you using File>Import? Photos does lots of scanning and checking during the import process. Many of us have found that importing more than 1000 pictures at a time is problematic. With an older machine and OS, that may be different.

Richard.Taylor wrote:

First, a caution: That choice to "Copy items to the Photos library" isn't there for any normal use. Photos is a database that connects pictures to edits, comments, dates, and everything that happens in Photos. The database depends on having full control of everything!. As you have it, moving an original or doing much of anything with the original files may totally disturb the database and make your Library largely unusable. We have seen this happen. Personally, I keep the originals in Finder Folders as backups on external drives, but they are not used by Photos.

A reference library is highly discouraged as it eventually leads to lost originals and a corrupt library. I'd follow Richards example.