In Atera you can set up a number of different contract types. A Remote Monitoring type contract bills the customer based on the number of customer Agents.
When creating a new Remote Monitoring type contract you can specify that the Agents being billed are Macs.
For more information on working with remote monitoring contracts in Atera see the Create a New Contract - Remote Monitoring KB.
Under the Add Contract form Select Mac Agents in the Count By dropdown.