SQL Monitor is licensed per operating system.
For clusters, each node requires a license. Virtual machines and test nodes also require a license.
You don't need to buy extra licenses for multiple SQL Server instances on a server.
When adding a machine which is part of a cluster or Always On Availability Group, all nodes in that environment are automatically detected and monitored, and each of these machines requires a license regardless of whether it is expected to act as a passive node. It is not possible to monitor only part of a cluster or Always On Availability Group.