How many devices can call for heat with multiple zones


I had thought that there was a maximum of 10 devices that could call for heat in the whole house, but this page suggests that it is actually 10 devices per zone controller.

My setup is working fine as it is, but I would like to add 2 more radiator valves calling for heat. As I have both a smart thermostat and an extension kit if they were both wired as zone controllers could I have more devices that call for heat, or is it actually a limit of 10 per house.