There is a newly released update for Hyper-V that will increase the support limit from 16 logical processors (cores) to 24 logical processors! Since we support up to eight virtual processors for every one logical processor (8:1 over subscription) this increases the maximum number of virtual machines from 128 virtual machines to 192! Keep in mind that if all of those virtual machines had two virtual processors the maximum number of supported virtual machines would be 96 (96 vm's x 2 vp's = 192).
BTW: If your server has 16 or fewer logical processors (cores) you don't need this update and I would advise not installing it as you should only install updates you need.
Link To The Update: Update for Windows Server 2008 x64 Edition (KB956710)
Taylor Brown Hyper-V Integration Test Lead http://blogs.msdn.com/taylorb