One important rule in server licensing (especially in the Microsoft environment) is understanding your peak capacity. Very simply, you must be licensed for the maximum usage of your server – the maximum number of running instances. Even if you, for example, only ever need one instance, but there is a possibility at some point there will be two instances run on that server at the same time – it needs to be licensed for two.
The number of instances that can be run on a server at one time will vary based on the version you have. For example, Windows Server 2008 Standard only allows one instance at a time, where the Enterprise version allows four and the Datacenter version is unlimited.
It’s very important to know what the maximum instances running will be in order to properly license and remain in compliance.