Wait a minute, Doc, are you trying to tell me that my mother has got the hots for me?

Great Scott!

You don’t “assign” cores to VMs. When you create a VM and give it vCPUs, you are telling the host how many threads it can use at a given time. The hypervisor (in this case proxmox) has its own task scheduler. As work comes in from the VMs, it assigns them to available threads on the physical processor.

For example, if you have a quad core CPU, you can create as many VMs as you have RAM and other resources for and give them ALL 4 vCPUs. So you can have a single 4c/4t CPU and 10 VMs with 4 vCPUs each. Proxmox will simply balance the load using its own task scheduler and assign work to the physical CPU based on load, available threads, etc.

If you want to assess whether or not your physical processor is overloaded, you need to look at the summary for the host and see if the CPU usage and Server load stats to see if you are overusing the CPU.