How many devices can call for heat with multiple zones

Hi

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. https://support.tado.com/en/articles/4689126-how-does-tado-control-heating-zones-how-can-i-change-the-zone-controller-of-a-room

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.