To play many purchases from the iTunes Store, you must authorize your computer using your Apple ID and password. (Authorization helps protect the copyrights of the purchased items.) You can authorize or deauthorize a computer at any time.

What does authorizing a computer in iTunes do?

When you authorize your Mac or PC, you give it permission to access your music, movies, and other content. ... You can authorize up to 5 computers, which means that you can play your content on 5 different computers.

What happens when you run out of authorized computers for iTunes?

It resets the authorization count on the iTunes Store servers. If you find that a computer has been deauthorized afterwards, unless it's reauthorized, it won't be able to play protected iTunes Store content from that account, or redownload it from the iTunes Store, or use the Transfer Purchases function.