Dr. Gianni,
You are quite correct. You install Windows Server 2012 as the host OS then, after adding the Hyper-V role, installed that same media and license code up to two times as virtual guests.
Do note the same licensing applies if you are using VMWare, CentOS, or any other hypervisor. You still have to buy a copy of Windows Server and you are still limited to two guests if you purchase Standard Windows Server.
JamesNT