When you have a picture with someone that Photos hasn't identified as a person, you can open the Info Window (⌘-i), and there will be a plus sign with a circle.

Click on that plus sign, and a circle will appear on the picture that you can move over the face you're wanting to tag.

(You may have to first click inside the circle to tell Photos that's what you're pointing at.) Then you type in the name, perhaps King Charles III, or Prince of Wales, and this picture will be added to the others with that name.
You can find more about the People scans here:
Find and identify photos of people and pets in Photos on Mac - Apple Support