Hi,
as others already point out your best option is to buy Microsoft Datacenter license ... one per hypervisor CPU/Socket allows you to run an unlimited number of instances of Windows Servers simultaneously in virtual OSEs (ESXi servers)
Any additional info about that topic is here:
Microsoft Licensing with VMware | Licensing Articles
Microsoft Volume Licensing Brief - Licensing Microsoft Server Products in Virtual Environments
Once you have all the necessary CPU licenses you have to activate deployed MS servers via MAK key or KMS server with appropriate keys given by MS/DELL.
With MAK you have to manually enter this key on every server separately ... with KMS you will need to install KMS server into your environment and activate him with given key.
Once its done your servers should find KMS server on network and activate automatically...for more details see:
