Depends on the wattage that can be handled most QUALITY thermostat will be able to handle 700 watts which is enough to control your heat source for several tubs so long the heat source is the same for each tubs.
In a rack setup it is and generally the probe is place on the middle shelf of the rack.
You can control several stand alone tubs as well, again depends on the wattage that your T-Stat can handle and the wattage of each UTH and all UTH MUST be the same size and wattage, otherwise there will be a differential in temps.
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