A. First, please remember that your Hyper-V parent partition should be kept as minimal as possible. You don't want to run application workloads or additional components, because any vulnerability in the parent could expose the guest OSs. Ideally, your Hyper-V hosts should be running Server Core with only the Hyper-V Role enabled. That being said, what can you do if you really want to?
Different versions of Windows Server 2008 and Server 2008 R2 come with varying virtualization instance rights. The standard editions allow one virtual guest as part of the Standard license. Enterprise edition allows four virtual guests and Datacenter allows an unlimited number. The rule for this is that the host is only running Hyper-V role and no other workloads. It can't be a file server and it can't run DNS—it should only be Hyper-V host. If that's the case, you can run the maximum number of guest instances allowable with the OS version (e.g. four for Enterprise). If you're running an additional workload on the host, you lose the right to one of the virtual instances. This restriction doesn't matter for the Datacenter edition, but for Enterprise means you could only run three virtual instances under the license, and none with Standard.