Updating faces has never stopped updating

It is not possible to disable the faces scan. Photos needs the recognised faces for many services, for the memories, for search, for the automatic classification of objects and scenes. But Photos will only scan each photo once, directly after it has been added to your library. Ignore the People album and do not add any names.

Remove the people albums you do not want, but keep at least one album in the People album, or Photos will start over with the classification of faces.

As far as I know, you can't turn it off.

A People album of just one photo of a person will always say that it's still working on it but it will never complete because it's really just waiting for more photos of that person to be loaded. So it's more of a poorly worded message than anything else going on.

Really, you should just ignore the section if you're not going to use it. Deleting the contents may just cause it to re-evaluate the faces thus wasting computer resources.

And consider providing feedback to Apple about how you think it should (or shouldn't) work:

https://www.apple.com/feedback/photos.html

The "People" album has never really worked. I spent hours manually tagging people in earlier iPhoto and Photos releases, and in Catalina / iOS 13, it all catastrophically wrong. Apparently, the key photos in the People album were causing syncing problems. So, I had to delete my entire People album anyway. Even when it existed, iOS couldn't search for People anyway. So, what was the point? The Photos situation on macOS and iOS is dumpster fire bad at this point. Syncing no longer works reliably from Finder.

Where are you seeing the message about scanning for people? Is it below the main People album, or below the albums of some people?

Below the main People album the message about scanning for people should vanish after a few month, when all photos have been scanned. Each photo will only be scanned once on import, and again, if you retouch it.

But below the albums of individual people the message about "updating people" may remain forever. Photos will be waiting for at least a second photo of the same person, so it can learn what the person looks like. You have to name at least two photos of a person before the message will vanish. And if you only have one photo of a person, and never will meet this person again, the message will stay forever (unless you use brute force and duplicate the one photo you have).