one running virtual machine. Windows Server® 2008 R2 Enterprise Edition includes use rights for up to four virtual machines. This does not limit the number of guests that the host can run; it means that licenses for four Windows guests are included. To run more than four you simply need to ensure you have valid Windows Server licenses for the additional virtual machines.
Windows Server® 2008 R2 Datacenter Edition includes unlimited virtualization use rights, which allows you to run as many guests as you like on the physical server running Windows Server 2008 R2 Datacenter edition.
Use Windows Server® 2008 R2 Enterprise or Windows Server® 2008 R2 Datacenter editions for all Hyper-V hosts. Work with your account executive to determine the point at which Datacenter server becomes cost effective once you have completed the host server sizing methodology and have determined how many guests you will run on each host.
Reference Microsoft Licensing for Virtualization.
Virtual Machine Guests Hyper-V greatly increases the scalability of guest virtual machines in comparison
to Virtual Server 2005. Hyper-V guests, when utilizing a supported operating system, are able to support the following options:
Verify the support level for multiple processors, large RAM, and so on, for each operating system planned to utilize those settings.
Since Hyper-V supports such large guest virtual machines, there is a much larger set of viable workloads that can be consolidated, including multi-processor, multi-core servers, servers with large disk or IO requirements, and so on.
While Hyper-V can support large guests, in general it is prudent to only configure each guest with the resources needed. This ensures that resources are available for other guests or future expansion. For example, it is not recommended to make all guests utilize four logical processors if that is not specifically needed for the guests. Additional resources such as processors, RAM, and so on, can easily be added if needed.