Ben Lovejoy, 9to5Mac, pulled a quote from this Mashable iPhone X review:

One important limitation of Face ID: It only lets you register one face. That may strike many as unnecessarily limiting since Touch ID lets users register up to 10 [sic] fingerprints, but Apple says it found the number of people who register more than one person’s fingerprints is miniscule.

Ben continues:

The idea that hardly anyone registers more than one person’s fingerprint didn’t ring true to me, and our poll shows that it’s not true for 9to5Mac users at least.

At the time of writing, the majority of our readers have more than person’s fingerprints registered for one or more of their iOS devices. Almost half (48.98%) have one other person registered, and a further 6.85% have more than two people.

I would certainly agree with this. I have my entire family “fingered” on my iPhone 6s, and the Touch ID performance is still very fast.

Ben’s take on adding a second face to the Face ID registry:

My guess is that doing all these checks for more than one person would make face-recognition noticeably slower than Touch ID, and Apple was concerned that reviewers and consumers alike wouldn’t respond well to that. That, I think, is the real reason Apple limits Face ID to a single face.

Interesting perspective.