Good question @Rachel2010! This will depend on the type of Protect Plan you are subscribed to. The Protect Basic Plan offers coverage per device, while the Protect Plus Plan will cover all of the devices at one location. As you stated each device is set up on a different Ring account, the plans on each account are not shared and belong to only their respective Ring device.
If the devices were setup on one account, you would have the option of subscribing to the Protect Plus Plan and covering multiple devices at once, among other great features. I hope this helps! 