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 devices:
You should typically connect the device to a PC that is itself an authorized device.
You are only permitted to deauthorize two (2) authorized devices per FiOS account per calendar year without connecting through your PC and you are only permitted to deauthorize and reauthorize the same authorized device two (2) times within a calendar year.