QlikView Server comes in a number of editions designed for different organizations and purposes. Upgrading is done through the license key.
QlikView Server license keys can be installed on as many servers as required provided that only the licensed number of QlikView Servers are running at any given time.
To connect to a QlikView Server (QVS), each client needs a Client Access License (CAL). The CALs are purchased with QlikView Server and associated with the server serial number.
You can read Qlik's license terms, as well as dedicated documentation for QlikView license metrics on qlik.com: