Number of Devices!

Try "Deauthorize all" and then reauthorize any computers you still want to use (applies only to computers, not iPhones or iPads). See the following from Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

To deauthorize a computer that you don't have access to, you have to deauthorize all of your computers. Then authorize each computer that you have again. You can deauthorize all computers only once a year.

1. Open iTunes.
2. Choose Account > View My Account.
3. Sign in with your Apple ID and password. Forgot your Apple ID?
4. At the top of your Account Information page, go to the Apple ID Summary section.
5. Next to Computer Authorizations, click Deauthorize All.
6. Authorize each computer that you use again.

iPhones are not iTunes authorized devices -- they cannot be. Only computers can be iTunes authorized devices.

iPhones are "associated" devices and are not affected by the 5 device authorization limit.

See the following for more information:Authorization vs. association - Apple Community