Can one thermostat be used to control two mats, why not more?

Yes, one thermostat can be used to control two mats, as long as the combined voltage and amps still meet Mysas requirements for energy. The number of mats connected to one thermostat is limited to two because the National Electric Code (NEC) has rules that limit the number of wires in a wire nut. The more wires in the nut, the more dangerous the connections become.