Device limits depend on the content. I've seen one book that was limited to 2 devices while the default is 6 devices (number 7 will trigger the error). Music seems to use 10 devices as the default. In the book details, you'll see something like (I'm assuming that if this line doesn't show, the limit is the default of 6):
Simultaneous Device Usage: Up to 4 simultaneous devices, per publisher limits
If it's any consolation, when I updated to Windows 20 2004, my copy of Kindle for PC decided it was no longer registered. I registered it and then checked Manage Devices and Contents. I had 5 Kindle for PC devices, 3 Kindle for iOS devices and two Kindle for Android devices (not to mention the 4 Amazon Prime Video devices. I removed all the Kindle devices and re-registered my PC, iPhone and Android tablet. I left the Prime Video devices since entering user names and passwords on the not so smart TV or the Apple TV is a PITA.
I did wonder why after the 2004 update, Amazon pushed an update to Kindle for PC despite not having "Automatically install updates when they are available without asking me checked".
All this so I could read
Og-Grim-Dog and the Dark Lord (Me Three #2).