When you start using Windows Phone 7, you will need to have and setup a Windows Live ID and associate your phone with it. This ID is required to access features like Marketplace, Xbox LIVE and Zune Pass from your Windows Phone 7.
How many Windows Phone can you associate with any given Windows Live ID ?
You can have a maximum of 5 Windows Phones associated with a Windows Live ID.
In fact if you already have 5 phones registered and you try to download an app with a 6th phone you may get this error message:
Your apps are already in use on the maximum number of devices. Use the Zune software on your computer to update your devices.
You will not be able to register a 6th phone. You will first have to de-register one phone from the Windows Phone app marketplace… and wait … it seems that you can de-register one phone only, in a span of 30 days. Which means that to remove another phone, you may have to wait for 30 days.
To remove your phone, open Zune software and sign in > Settings > Account > Computer and Devices. Here you may remove and de-associate your Windows Phone 7 with your Windows Live ID.
Now you can register your new phone. In fact the next Windows Phone you use with the app marketplace will be automatically registered!
Windows Phone 7 Marketplace FAQ may also interest you!