From http://download.microsoft.com/download/3/D/4/3D42BDC2-6725-4B29-B75A-A5B04179958B/WindowsServer2012VirtualTech_VLBrief.pdf
For customers that want to have a highly virtualized environment,
Windows Server 2012 R2 Datacenter edition is the right edition to use.
Windows Server 2012 R2 Datacenter edition will allow you to run
unlimited virtual instances with each license on a single server. This
allows you to grow as much as you need to, and with no limit there is
no need to track how many instances you are running to ensure that you
are compliant.
Also, read up on Automatic Virtual Machine Activation (AVMA), new to Server 2012:
http://technet.microsoft.com/en-us/library/dn303421.aspx
Practical Applications [of AVMA]
On virtualization servers that are activated using Volume Licensing or
OEM licensing, AVMA offers several benefits.
Server datacenter managers can use AVMA to do the following:
• Activate virtual machines in remote locations
• Activate virtual machines with or without an internet connection
• Track virtual machine usage and licenses from the virtualization
server, without requiring any access rights on the virtualized systems
There are no product keys to manage and no stickers on the servers to
read. The virtual machine is activated and continues to work even when
it is migrated across an array of virtualization servers.
Service Provider License Agreement (SPLA) partners and other hosting
providers do not have to share product keys with tenants or access a
tenant’s virtual machine to activate it. Virtual machine activation is
transparent to the tenant when AVMA is used. Hosting providers can use
the server logs to verify license compliance and to track client usage
history.