A server is any Unix host - physical or virtual - where you have the QAS client package(s) installed and the QAS code being executed.
For each machine QAS is installed on (and therefore presumably executes on) there needs to be a server license. All servers in an environment where some user could potentially log in, and execute QAS code, must be licensed individually. This usually corresponds to a computer object in Active Directory (AD).
In a cluster situation, if multiple machines use one computer object, and they use it at the same time, they each need a license (Load balancing situation). If only one machine uses the computer object at any one time, then only one license is needed for the set of machines (failover situation).