To how many Microsoft Windows Server licenses should I subscribe per virtual machine?

The total number of Microsoft Windows Server licenses you subscribe to is proportional to the number of vCores in your virtual machine (VM). The number of licenses is non-divisible.

One license can be used for up to 4 vCores. For every 4 additional vCores, you must subscribe to one additional license. 

This means that if your VM contains 4 vCores, you must subscribe to one Microsoft Windows Server license. If your VM contains 5 vCores, you reach the next tier, and you must subscribe to 2 licenses, and so on.