ServerInternals is licensed on a per installation basis. When you purchase your licences, you'll recieve one serial number for each licence purchased, and each serial number is used to activate the software on a single computer.
Once activated, ServerInternals can then be used to connect to an unlimited number of servers to display diagnostic information.
No additional licence is required for the systems to which ServerInternals connects via WMI or WinRM - a licence is only required for each computer running the ServerInternals software itself.
There are two licence types available. The full licence, which is intended for workstations, allows the entire ServerInternals suite to be installed. The agent-only licence, which is intended for servers, only allows the collection agent to be installed.
Typically, an organisation will require one full licence for each individual who is using the software to understand system behaviour and diagnose issues, and one agent licence for each server on the network.