When you first watching / downloading with a device, it will be automatically registered and displayed in the “Manage Devices” section under Settings / My Account on the following platforms: 

• Now TV App/Website (“Manage Devices” is under Settings) 
• Now TV Smart TV Apps (“Manage Devices” is under Settings) 
• Now TV Shop website (“Manage Devices” is under My Account) 

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Each Now ID account can register up to 5 devices at a time^.  If you attempt to play or download content from a 6th device (having 5 devices already registered), your earliest registered device will be automatically deregistered and removed, and the 6^th device will then be registered automatically. You can then play or download content from the 6th device without hustle.
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^Now TV Set-Top Box is not counted as a registered device.  

There is no limit to the number of times you can change your registered devices.

Each Now ID account can register up to 5 devices at a time^, and concurrently stream or download from up to 2 of those. 
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^ Now TV Set-Top Box is not counted as a registered device.  

Yes. The above-mentioned registered devices limit applies to each Now ID account, including the “Multiple User Profile” (each Now ID account can have up to five profiles).  Thus, the registered device limit is shared among all users under a single Now ID account, which allows for registering up to 5 devices at a time, and concurrently streaming or downloading from up to 2 of those. 
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^ Now TV Set-Top Box is not counted as a registered device. 

No, all web browsers would be registered as 1 device, with the device name “Web”.