What is a reasonable number of virtual servers on a physical server with roughly the following specifications:
- 2 x AMD Opteron 6128 CPU's
- 16 Gb RAM
- 3 x 1500 Gb hdd RAID 5
Hyper-V will be the hypervisor
It really depends on what types of VMs you're going to be hosting. If they're basic file/print, email, web servers I'd probably stick to about 5, maybe 6.
You're really limited by your RAM, If its a budget concern I'd consider dropping one of the processors to add some additional RAM.
It really depends on what you want to do with these virtual servers, from my experience the biggest limitation you'll have is the 16Gb of RAM
So let's say that you have 512Mb servers (nothing to sniff at), that'll give you a max of 31 servers (I know the numbers are 32, but I like to leave some memory for the KVM machine to operate)
As a rule of thumb, consider your memory requirements and that'll give you a very good idea of how many servers you can run, since CPU time can be shared, unless you're running some very heavy computational clusters it shouldn't be a limitation.