Hello!
In the Device Class billing model, you are charged based on the average daily number of MEAs a device sends. You are correct that a Device Class C has a maximum average daily value of 1440 MEAs.
A “Device” in this billing model is any inventory object that has at least one MEA attributed to it in a given month. Inventory objects that do not have any MEAs attributed to them in a given month are not considered billable devices. In addition, inventory update requests do not count against the device class billing model.
To track your Device Classes, I strongly suggest using the Usage Statistics Application, as this gives you a clear overview of all devices in addition to allowing a drill-down into each device. Check it out here!