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Summary
This document discusses how Microsoft Windows Server 2012 is licensed when used with virtualization technologies like VMware ESX/ESXi, Microsoft System Center 2012 Virtual Machine Manager, or Parallels Virtuozzo.
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Windows Server Licensing on top of VMware’s ESX/ESXi hosts
If a server is running ESX/ESXi as the virtualization technology, then Windows Server is not deployed as a host operating system in the physical OSE. However, a license is required for every physical processor on the server and every instance running in a virtual OSE should be appropriately licensed (Standard edition will allow up to two virtual instances with each license and Datacenter edition will allow an unlimited number of virtual instances with each license).
If you have assigned a single license for Windows Server 2012 Standard to a server running ESX/ESXi, then you may run two instances of Windows Server 2012 Standard at a time. The right to run an instance of Windows Server 2012 in the physical OSE cannot be used in this case since ESX/ESXi runs on the physical OSE (and as a result, Windows Server 2012 cannot be deployed as the operating system on the physical OSE.)
If you have assigned a second Windows Server 2012 Standard edition to the server running ESX/ESXi, then you may run up to four instances at a time of Windows Server 2012 Standard. You may not run a fifth instance under the same license since that right requires that the fifth instance be running hardware virtualization software and software managing and servicing the OSEs on the server.