Yesterday I had called Microsoft support and they asked for the license No and Authorization Number.
The agent communicated that you when you purchase Win 2012 Server 2012 Data center Edt you get the right to install unlimited VM’s. As per the PO the customer has purchased only 6 licenses but against along this order Microsoft has provided them license keys of different version of Operating Systems as MAK/KMS Keys and we have installed the Virtual Machines using the same keys.
Therefore each server needs a separate Windows Server Data Center license to give you the ability to run multiple guest Operating systems. From a compliance perspective you need separate license for each server whether it’s a Hyper-V, or ESXi Virtualization.
Secondly when you purchase Data Center Edition Microsoft gives you access to MAK in order to activate your VM’s. The MAK/KMS keys microsoft provides are for N-1 Operating system if you want to get the keys for like 2003 server you can contact microsoft.
Please find attached relevant documents.