https://techcrunch.com/2017/09/14/apples-face-id-will-support-only-one-registered-face-per-iphone-at-launch/

Apple’s forthcoming FaceID technology will support just one face registered per device, at least initially. This has been reported by multiple people who spoke to Apple at this week’s iPhone event, and confirmed separately by TechCrunch, but it’s worth noting because it could change some user behavior regarding the device.

Face ID supporting just one face makes a lot of sense on the face of things (sorry), since unlike with Touch ID, users have only one face to use for their unlocking purposes. Touch ID ostensibly allows multiple fingers to be registered because you have quite a few digits of your own, and depending on how you’re using the device or in which pocket you keep it, one might be more convenient to use than another.