Pricing metric is per system. The system can have a maximum of two physical processors (sockets), unlimited number of cores and/or virtual guests.So you don't have to purchase any licenses but you have the option of purchasing support. The licensing metric is per system (so the number of cpus, cores or virtual machines does not matter). The one I quoted above is for the "limited" suport version that has a maximum of two sockets per system, there is also another option for an unlimited number of sockets per system. And if I recall correctly, you do not need to purchase support for the OVM manager machine.
There is no license cost associated with Oracle VM. Pricing reflects $0 for license, and all charges are related to services.