If multiple devices share a Store Apple ID they will show up in Apple ID Devices. They will also show in iTunes (for that Store Apple ID), Apple’s current documentation states iTunes is the only way to see and manage this list. “You can have ten devices (no more than five of them computers) associated with your Apple ID and iTunes at one time."
And you thought iTunes was dead!
You have to remove devices manually from this list after you stop using them. If, like me, you use the same Store Apple ID on family devices it’s easy to hit the limit.
The interesting bit is these two lists are different and they don’t synchronize. They are presumably on two different databases.
The applied.apple.com list is current and shows 8 devices. I think if you sign out of a device you’re not using this list will be updated.
The iTunes managed list is not updated when you sign out of a device. You have to update it manually. I think it still supports iPods. It had one of our devices that was no longer active on it, but it also had an old iPhone 4 we use for music only that runs iOS7 
iPod support explains why the iTunes managed list can’t be automatically updated. I don’t know what happens if you exceed the limit on one list but not the other.
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 The iCloud My Devices display supports “iOS 8, macOS Yosemite … or later …”