A:Answer I don't believe there is a limit to the number of devices that can be alerted. It sends alerts to the Nest account that is logged in. My wife is the "owner" of the Nest account where all our Nest devices are configured. I have my own Nest account but am added as a family member. Alerts get sent to both. So you can add multiple users, and if those accounts are signed in, alerts will go to all the devices.
The Nest Protect can go on a wall or ceiling. There are instructions for both wall and ceiling mounting, including angled walls/ceilings.