Solutions can have different licensing models depending on the specifics of every solution.
Every solution that integrates its license management in vCenter Server is licensed on a different basis. Some solutions can be licensed on per-instance basis, others can be licensed on per-virtual machine basis. For example, vCenter Site Recovery Manager 5.0 is licensed on per-virtual machine basis. A license key for Site Recovery Manager 5.0 defines the number of virtual machines that you can protect.
For information about the licensing model of a certain solution, see the documentation for that solution.