I have many more faces than are showing in Library-->People, yet they're not all being shown so I can add their names. Why isn't Photos on Mac finding more faces for me out of thousands of photos?

Photos initially only showing a selected group of the people found in many photos. If you want to see more people in the People album, you have to add these people manually to the People album. Find a photo of a missing person, where the face is clearly visible. See: Find and identify photos of people using Photos on Mac - Apple Support

Some photos may contain people who are not automatically named. You can name them manually.

1. In the Photos app  on your Mac, double-click a photo to open it.
2. Do one of the following:

• • Choose View > Show Face Names. Click the name (or click “unnamed”) under a face, type a name, then press Return (or choose a name that appears as you type).
• Click the Info button  in the toolbar. Click the Add button , drag the circle to position it over the face if necessary, click the Name field, type a name, then press Return.
  • You can click the Add button  multiple times to identify multiple faces in a photo. To remove a name that you added to a face, click the Remove button  on the person.

To stop showing face names in photos, choose View > Hide Face Names.