Only one element runs at a time no matter how many heaters you link. So the limit would be determined by the resistance of the circuit. Eventually you’d max out the 30amp circuits lengthand how many can you connect in a row before you max out the breaker?
As each thermostat is thermally satisfied it switches the power to the next thermostat snd subsequent element. And so on down the line....
Electrically the heaters are acting like one big water heater. Make sense ?
It’ll never pull more amps than the elements wattage divided by the voltage applied. It’s still operating as a non-simultaneous element fired heater.