Windows Server 2012 STANDARD provides TWO Virtual Operating System Environments (VOSE’s) where as Windows Server 2012 DATA CENTER provides an unlimited (well the physical limit of 128) VM’s. So the math to decide what you need to buy for you project should be pretty straight forward.
Note that if your server has MORE than two CPU’s (i.e. 3+ CPU’s – not CORES but physical CPU’s) then you have to use DATACENTER as STANDARD only allows for two CPU’s.
Below is some math from HERE that “proves” if you have less than 10 VM’s you likely should stay with Windows Server 2012 STANDARD:
There are two options; do you go with the Standard or Datacenter editions of Windows Server 2012?
The Standard edition covers 2 VOSEs, but the customer wants 10 VMs running Windows Server Standard 2008 R2. A single copy of WS2012 Standard will not suffice. 5 copies are bought to provide the 10 (5 * 2 VOSEs) VMs with licensing. Based on USA Open NL pricing the licensing of these VMs will cost $882 * 5 = $4,410.
The Datacenter edition of WS2012 gives unlimited VOSEs and covers 2 CPUs in the host. This solution will require a single Windows Server 2012 Datacenter license which will cost $4,809.
Decision: If you will not go over 10 VMs on this host then Windows Server 2012 Standard edition is the way to go. If you estimate that there is a good chance of the VM numbers growing then spend an extra $399 and pick up the easier to account-for Windows Server 2012 Datacenter with it’s unlimited VOSE rights.
10 is the magic number using USA Open NL pricing. Once you reach 10 VOSEs on a 1 or 2 CPU host, you need to consider the Datacenter edition because it is cheaper once you hit 11 VOSEs.