This is completely unrelated to Prime. Amount of people interested will drive the price.
It's $150/yr, 6 total spots. $150/6 = $25 ... not too shabby. Even at 4 people, it would be $37.50 for the year each.
I have an open dialogue opened with Amazon now about understanding if there's a cap of devices allowed. If there is a cap, is it for the overall family plan, or per member of the plan. Will report back once I have an answer there.
The amount of devices don't matter too much to me since I basically only use it on my phone for Bluetooth. I'm still interested but I have prime music now that I don't use all that much. I'm not sure if the upgrade to unlimited will be worth it for me. I'll keep checking back.