If you’ve subscribed to iTunes Match, or used any of your computers or iOS devices to re-download or automatically download purchased content, these devices become registered to your iTunes Store account. While there’s normally little need to worry about this, Apple does enforce a maximum limit of ten (10) devices that can be used to access iTunes in the Cloud with a single iTunes account, which can become a problem for larger families and those who regularly upgrade or replace devices.
Fortunately, you can easily remove these devices from your account if you’ve hit the maximum or simply want to clean out devices you’re no longer using. Simply go to your iTunes Store account settings by selecting View My Account from the Store menu in iTunes, and then choose “Manage Devices” from the Account Information screen.
This will display a list of all of the devices currently associated with your iTunes Store account for use with iTunes in the Cloud features with a “Remove” button beside each one. Keep in mind that removing a device in this manner does not automatically disable features such as iTunes Match on the device—you’ll still need to do that directly on the device.