You can add up to five of any of the following types of devices:
FiOS TV (all Set-Top Boxes in your home count as 1 device)
Wireless Cell phones (Supported models)
Windows® PlayReady® compatible portable Media Players
After reaching the limit of five devices you are then required to deauthorize an existing device before you can add a new one.
To authorize and deauthorize portable Multi-Screen devices:
You should typically connect the device to a PC that is itself an authorized device.
You are only permitted to deauthorize two (2) Multi-Screen authorized devices per Multi-Screen account per calendar year without connecting through your PC and you are only permitted to deauthorize and reauthorize the same Multi-Screen authorized device two (2) times within a calendar year.