I've always wanted a two-factor lockscreen, so that I can scan my fingerprint and then type a PIN. A face scan would work even better in this context.
how many usernames do you need in your phone?
Android supports multiple "profiles", so it's convenient when you have a shared device such as a family tablet.
Just one, but that's the point. Don't present the person holding my phone with a chance to enter a password or passcode unless you know it's me, by scanning my face.