What happens when someone has less than the usual number of fingers?

Our internal hand tracking model supposes that a hand has four fingers and a thumb, so if it doesn't see what it expects, it will attempt to compensate.

This means that the missing finger would probably appear in the tracking as being curled out of sight, or occluded by another finger. The software might also sometimes mix up the missing finger that it thinks it sees, and an existing finger. (For example, if you're missing your little finger, from some angles the software may sometimes mistake your ring finger for the missing little finger.)