Stamps are discrete units of core solution components that support a defined number of devices. Each copy is called a stamp. or scale unit. For example, a stamp might consist of a set device population, an IoT Hub, an Event Hub or other routing endpoint, and a processing component. Each stamp supports a defined device population. You choose the maximum number of devices the stamp can hold. As the device population grows, you add stamp instances rather than independently scaling up different parts of the solution.

https://learn.microsoft.com/en-us/azure/architecture/example-scenario/iot/iot-move-to-production