You should ask this in a Microsoft forum. Of course every vSphere Admin can answer this as well because microsoft doesnt make a difference when speaking of Hypervisor solutions.
A Windows Server license is bind to the HW when installed/powered on. If you never migrate (vMotion , HA, $Whatever) the OSI away you licensed this Host and its CPU and youre done.
If you change the OSI(VM) to another HW:
I you have Server Std. OEM its not allowed. Solution is to buy licences for all other Hosts/CPUs as well
If you have Server Std. OpenLIC/Value you can reassign the license every 60 Days. If you would like to vMotion/HA earlier you have to license all the other Hosts/CPUs as well,
If you have Server DataCenter OEM/OpenLic/Value and you have licensed all effected Hosts than you can migrate and move around the VM every second if you like.
If you have a 3 Node Cluster and HA can restart a VM on every of the 3 Hosts than you need a Windows license for ever Host.
If you have Application as MS SQL/Exchange than the next challenge is coming
Regards,
Joerg