How many versions are Windows 10 are?

The version of Windows 10 that we are all playing around with right now in test form is Windows 10 Desktop. This will be the version that runs on Intel-based PCs, laptops and hybrid/2-in-1 devices.
The version of Windows 10 that will be available in test form starting in February 2015 is Windows 10 mobile. That version is
optimized to run on ARM-based Windows Phones (from Microsoft and other OEMs), as well as on ARM- and/or Intel-based small tablets, ideally under eight inches in size.
There’s a Windows 10 “Industry” SKU for devices like point-of-sales terminals, ATMs and kiosks.
There will also be a custom version of Windows 10 inside the Surface Hub — the conferencing system that Microsoft announced last week.
There’s Windows 10 “Athens,” which also will work on ARM or X86 cost- and resource-constrained devices. Athens is based on the Windows 10 common core and will support the “Universal Apps” model.
There is apparently a different custom version of Windows 10 that will power the HoloLens augmented-reality goggles.
There will probably be Windows 10 embedded versions of the OS that will run inside cars and fridges etc
There will inevitably be versions of Windows 10 at the Server Level – I am assuming Windows 10 Server.