Q: I'm planning CPU requirements for a Hyper-V host--do I need to reserve processor resources for the Hyper-V host?

A:  Typically no. When thinking about processor resources, it will be the virtual machine (VM) virtual processors performing most of the work. Provided the Hyper-V host isn't running any other role or application, isn't running antimalware, and guest OSs are running the latest Integration Components and you aren't using the legacy network adapter for normal traffic, then the Hyper-V host isn't really doing processor tasks and is really just facilitating IO.

Even very large IO loads still don't cause much processor use on the host, as the VM virtual processors do most of the work.

The hypervisor makes sure the host gets what it needs, and so there's no need to reserve a specific amount for the host specifically. Focus on ensuring resources to meet the needs of the VMs.