Syncrify licensing is based on client machine, you don't pay using the Syncrify server. However, when you purchase a client license, it is tied to only one server. This means you can use one client-machine license with only one server.
Consider the following example:
You want to backup client machine A
You have two machines running Syncrify Server (S1 and S2)
You want to backup machine A to both S1 and S2
You will need to purchase two client-machine licenses. Using the same license key for both S1 and S2 is a licensing policy violation.