Itunes says I need to deauthorize a computer but only 1 is authorized

You can see a list of associated devices but not of authorized devices. See the following for information about both: Authorization vs. association - Apple Community

The simple thing to do is just "Deauthoirze All" and then authorize any computers (not iPhones/iPads) that you still want to use. See the following from Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

For Windows PC:

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.

For Mac computers:

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 the Apple Music app or Apple TV app.
2. Choose Account > Account Settings.
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.