Microsoft have quietly increased the Windows Store device limit up from 10 to 2000. Every Windows device you use with the Windows Store (including Xbox One) is registered with the store and once you hit the device limit you had remove an old device before you could download on a new device. Windows 8.1 had a limit of 81 devices but with Windows 10 the limit was reduced to 10. The new device limit is 2000 which it seems like plenty, even someone like me that frequently tests new machines.
I have to say I don’t remember seeing a warning recently so the device limit could have changed some time ago. If you go to account.microsoft.com/devices/content you can have a look at your own device count.