You need one license for each machine you want to install Cerberus FTP Server on. A license cannot be installed on more than one machine at a time.
Unique licenses are also needed for disaster recover (DR) machines, testing, development, staging, and quality assurance installations. However, we provide a 50% discount on these types of licenses when purchased in support of a production license.
If you wish to cluster multiple machines together for high availability and load balancing you will need a full production license for each machine. Only the non-production licenses noted above qualify for a discount, and only in support of one or more production licenses.