That all depends on the function that the Virtual Machines will be providing, how much vCPU and vMemory will each guest get, how utilized will the guests be, its always a big "it depends" answer and there isn't a 1 size fits all solution to this question.
If you work with a VMware partner, they can run VMware Capacity Planner and give you and idea of what you would need to support your virtual machines if you were p2v'ing physical servers.
