This is not very easy to understand and it depends on your environment and how many physical hosts you have.
You only have 2 licensing options. Standard or Datacenter. (There is no Enterprise license for 2012.)
All licenses are bound to the physical server (ESXi hosts in this case). Not the virtual machines.
1 license covers 2 physical CPUs. (Cores doesn't matter)
With 1x standard license you are allowed to run 2x Virtual Server 2012s on 1 host.
So if you for example have 6x Server 2012s you need 3x Standard licenses on 1 host.
If you run 21x Server 2012s you need 11x Standard or 1x Datacenter on 1 host.
With 1x Datacenter license you are allowed to run unlimited Virtual Server 2012's on 1 host so you have to calculate what your price is for x Standard vs 1x Datacenter and depending on how many Virtual Server 2012 you plan to install there will be a breaking point for where the Datacenter license is "best buy". I think the breaking point is somewhere around 7x Server 2012s but don't remember exactly...
If you run several hosts with HA you need some more calculation since the Server 2012 can vMotion to another host and licenses can't "float" so you need full licenses for both host 1 and host 2.
If you use the Datacenter license you can run unlimited Virtual Server 2012s on 1 host. If you have 2 hosts you need 2 Datacenter licences for unlimited.
Please feel free to ask any questions and I'll try to help you. Your MS reseller should be able to help you as well but I my case they where not 100% sure so I had to read the licensing guides over and over again to finally get it :smileyhappy:
Another guide which I prefer. Page 7-13
http://download.microsoft.com/download/7/7/0/7707E736-4557-4310-9709-87358F7E6D1A/WindowsServer2012VirtualTech_VLBrief.p…