Whenever I open the people tab I see the notification “Photos will continue scanning your remaining photos when you’re not using the app and your Mac is connected to power.” So, does the app need to be open or not. I never see any progress.
You just have to launch Photos after you start up your Mac to launch the background processes, then you can close the Photos window. when you are indoubt, if the Photos background processes are still active, restart the Mac, then launch Photos again.
How long have you waited already? Photos is a bit lazy about showing the progress of the faces scan. In my Library it has been showing "zero photos scanned" for several weeks, and then suddenly all photos have been scanned. But it did not really matter, the previously recognized faces have still been there, when I
viewed the People album.
Apple Photos app scanning
