Cerberus FTP Server licenses are per machine licenses. You will need one license for each machine you wish to install Cerberus FTP Server on.
Each Cerberus FTP Server subscription includes access to the product, software updates, and technical support. Licenses are sold as three-year subscriptions with annual billing.
If preferable, there is also an option to pay in full for all three years (you would then receive a three-year key versus a one year aligned with annual billing).