Can facetime be limited to one number?

Hey there, GoalTorrent,

Thanks for visiting the Apple Support Communities. With Screen Time, you can set up parental controls on a youngster's devices to limit what they have access to. You can find more information on what this entails here: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

While there isn't an option to exclude all contacts but you, you can set yourself as the only contact on your granddaughter's iPad, and can use the other parental control features to restrict her ability to make changes.

If you'd like to see additional features as part of Screen Time and parental controls in the future, please let Apple know here: Product Feedback - Apple

Have a wonderful day.