The Microsoft Hyper-V team this week posted a new article detailing some really great information about Hyper-V density, virtual processor to physical processor ratio and a bunch more information that is invaluable to Hyper-V administrators interested in ensuring that their hosts run in the most efficient - and maximum - way possible.
I have been using 8 core machines with Hyper-V hosting a maximum of 15 machines (30 Virtual Processors) per machine. I notice that the processor utilization is less than 20%, maybe the load is not much. But according to the document, I can load VMs utilizing 128 Virtual Processors on these machines and I am utilizing only 30. I do feel the document makes sense, however I remain skeptical in loading more than 50% of what Microsoft says is supported.
I agree that using published maximums as a baseline s probably best avoided. First, it leaves you no room for growth and it's unlikely that a less than optimized system could use it all anyway. You may be happy to know that the upcoming Hyper-V 3.0 increases some of these maximums, so it might help you feel more comfortable moving beyond the 50% point.