iTunes

From the linked article:

If you run out of authorizations, deauthorize all

If one of your 5 authorizations is on a computer that you don't use anymore, deauthorize it. If you sold or gave away a computer that's used one of your 5 authorizations, you need to choose Deauthorize All on the Account Information page.* Then authorize each computer that you still use.

* Account > View My Account...

See also Authorization vs. association - Apple Community.

tt2

Open the menu item Account > View My Account..., there should be a deauthorize all button there. Once used you cannot use this feature again for a year. You mentioned 8 devices, those would be associations not authorizations. You may have some you no longer need. See the link I posted for all the details.

tt2

One you use Account > View My Account... and sign in with your Apple ID you should see a page that looks like this.

tt2

Try iTunes > Preferences > Advanced and tap Reset Cache. Then try viewing your account page again.

Otherwise what version of macOS are you running? El Capitan to High Sierra you should reinstall iTunes 12.8.2.3. If it is Mohave reinstall the operating system to reinstall iTunes.

tt2