I haven’t worked a ton with Windows Server 2012, but recently while debugging an issue for a client relating to performance on a Hyper-V hosted VM, I was stumped why I was only seeing 1 CPU when I had added 4 vCPUs to the VM.

First, I was able to confirm that the Guest OS was seeing all 4 CPUs by looking in Device Manager.

It turns out that by default (or at least in my case) the CPU utilization is displayed in an aggregated way, making me think that it was only using 1 CPU:

By right clicking on the CPU graph and selecting Change graph to… Logical processors you can see all the CPUs.