Windows activation has been 'tied to the hardware' for a long time, since XP if memory serves. Assuming you're not using OEM or volume licences, that is. Windows 10 is no different.
Running Windows 10 in a guest VM means it's tied to the virtual hardware in the VM including the virtual CPU ID. But you should be able to change things like the allocated memory, virtual disks and so forth without triggering a reactivation. If you do trigger one, it might re-activate online, it might require a call to Microsoft.
Because VMs can be cloned and copied you obviously can't have 20 VMs all using the same licence. However since the CPU ID will be different for each, it almost certainly wouldn't work anyway.
There is a discussion of it here:
http://www.tenforums.com/windows-insider/38992-question-about-licence-activa..."