What is the exact text of the error message, and what are you doing to trigger it? Perhaps you are confusing computer authorization with device association.
Up to 5 computers can be simultaneously authorized for each account. iPods & iOS devices are limited to DRM protected content from up to 5 different accounts. There is no limit to the number of devices that you can load up content with using iTunes. You may deauthorize computers individually using iTunes on that computer, or deauthorize all and then reauthorize those you wish to use thereafter. Upgrading or reinstalling Windows without deauthorizing first will lose one of your authorizations. You may not use deauthorize all again for a period of 1 year. Apple TVs are not counted towards this limit.
HT1420 - iTunes Store: About authorization and deauthorization
A mix of up to 10 computers and iOS devices can be associated with your Apple ID for iTunes Match, Apple Music, or automatic downloads of past purchases. There is a 90-day limit before a computer/device can be switched to another ID. No more than 5 computers can be associated with your account. Apple TVs are not counted towards this limit.
HT4627 - iTunes Store: Associating a device or computer to your Apple ID
tt2