“Six” is the number.
Some other important points:
- Any family may only have ONE organizer
- Any member may only be in ONE family
Set up Family Sharing - Apple Support
Also, to setup for your 12yos:
Note that their Apple Accounts must be first setup from the adult family organizer’s device … and then setup the children’s devices.
Create an Apple Account for your child - Apple Support
A child must ALWAYS be a member of a family. (i.e. no Apple Orphans) A family can not be disbanded if it has a child as a member.
A child who is already a member of another family must be invited into your family before her previous family is disbanded. (these instructions must be followed EXACTLY)
Move a child to another group using Family Sharing - Apple Support