1. Use Windows Server 2019 - optionally 2016
    Today you shouldn't use anything older, because it will hit end-of-life soon
  2. Make sure you patch the servers regularly - we recommend quarterly or sooner in case of serious security risks
  3. Always run a modern server anti-virus and backup solutions on the system
  4. In terms of licensing we really urge you to use Windows Datacenter SPLA for your host, as that allows you to run unlimited VMs on it without extra cost. So you can create things the way it makes sense and not be restricted by licensing considerations. 

Note that for HA (High-Availability) there are some more things you can do, but as for the Divide and Conquer model we'll keep that simpler.