I'm confused, how do I identify whether the physical RAM is sufficient for the number of virtual machines that I wanted to run on any given box.

It's become pretty easy now. All you have to do is download the excel sheet from here. Enter the amount of physical RAM and the memory requirement for the number of virtual machines you want to run. Say, you would like to run four virtual machines with each 4GB assigned on a 32GB physical machine, the excel sheet would calculate the following things "how much memory the host need", "Hyper-V service required" and let you know the remaining physical memory.