Apple TV plus devices

Family Sharing is about user accounts (Apple IDs), not devices per se.

The maximum group size is 6 users.

The maximum number of simultaneous streams for Apple TV+ is 6 per subscription (i.e. the whole group).

Each user can have multiple devices, which will be fine, as one person will typically use no more than one device at a time for watching streams. Still a lot more generous than most other content providers.

7–12 users can form 2 Family Sharing groups, and have 2 subscribers that share Apple TV+.

It's not allowed. Someone else in the home would need a second subscription for the extra devices.

Good Luck!