if they are all the same heating element, same wattage, and in the same conditions then yes. for example, 3 (or more) racks stacked or side by side can all be controlled by the same device.

you wouldn't want to control one sweaterbox rack and one 3' rack with the same thermostat since the wattages are so different. and you wouldn't want to control two identical racks in different locations with the same thermostat since the ambient temps will be so different.

Hope that helps,

JonV