The ‘People and Pets’ collection in Apple Photos will group all pics of people (and pets) it finds, working away in the background. You have to name them initially. I just checked my wife and it has found 6,383 photos of her in my 86,000 pic library. Many of these she is one of a group or in the background.
It works pretty well but makes mistakes, which can be corrected, and it sometimes treats someone as two different people and you can merge them. I do not name any photos so it does not rely on that…it all works by facial recognition. It can take a while depending on size of Library and how much time the machine is on and Photos app not in use. If you use iCloud, People and Pets are sync’d across devices