Nested virtualization and subscription

Given that Proxmox can be installed inside a third party hypervisor (see Nested Virtualization - Proxmox VE ) how should I calculate the number of physical sockets in order to purchase the right subscription?

Should I count all the physical sockets of the hypervisor (even if they server lots of other VMs) or should I count the vCPUs assigned to the guest Proxmox server?

Regards