SQL Backup is licensed per server which means a distinct system running a Windows operating system, whether physical or virtual. You will need one license per server even if you have multiple SQL Server instances running on that server. If your servers are clustered, you will need a license for each node running SQL Server.
SQL Backup consists of two parts: the GUI and the server components.
From the GUI you will monitor your backup and restore jobs. The GUI doesn't require a license and can be installed on as many machines as you need.
The server components should be installed on each SQL Server instance (or on each node of a clustered instance). These parts require a license and contain the SQL Backup extended stored procedures and the SQL Backup Agent service. You can install only one version of the SQL Backup server components on each SQL Server instance.