Once upon a time, the way to decide which Microsoft Windows Server license to get was only a matter of checking the servers number, a bit of their configuration and features you wanted to get. Basically, the Standard Edition was good for anything that had less than 32GB of RAM and/or wasn’t used in clustered configuration ; else you would require the Enterprise Edition. There were also options for Web, Datacenter, Storage Edition but I never had to look at those.
Virtualization, Cloud computing and IaaS have turned servers into instances with virtual resources which made those licenses quite inapplicable ; especially OEM license and CPU limitations. Here’s the way I understood the Microsoft documentation. It may enable you to understand and decide which licenses to use in your particular case. I’m writing it with VMware vSphere implementation in mind, but it should apply to apply virtualization technology.