Windows Product Licensing

Windows Server licenses are one per physical or virtual machine.

Then on top of that, if you are using some types of client services you need a client license per concurrent client use for each server.

If you have five servers, talk to an authorized Microsoft retailer about a volume licence, which should be cheaper. As mentioned, you also need CALS (client access licences) to access resources on the server from another computer. These vary depending upon what access is required (as a web server, as a file server, as a terminal services server, etc.). A server licence used to come with 5 CALs for file sharing access, but I'm not sure if it is still the case.

Microsoft licensing can be very complicated; to get the best deal you must discuss your requirements with a good retailer.