On its corporate blog the Microsoft Windows Virtualization Product Group informally releases some details about Virtual Server 2005 R2 (with and without SP1) configuration limits:

• Virtual Server R2 on 32bit Windows hosts – 64 concurrent virtual machines
• Virtual Server R2 on 64bit Windows hosts – 64 concurrent virtual machines
• Virtual Server R2 SP1 on 32bit Windows hosts – 64 concurrent virtual machines
• Virtual Server R2 SP1 on 64bit Windows hosts – 512 concurrent virtual machines

This is critical information to be considered before starting any virtualization project. VMware discloses limits of its virtualization solutions as well in a much more detailed document, updated at new releases time.