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No, you’ll never need an external supply even if you’re using dozens of 5TM soil moisture sensors. The reason is that the code I posted above will only power one sensor at a time. All of the other sensors are basically sleeping and using virtually no power until they are “woken up” one at a time by the call to their SDI12 address. One 5TM sensor only draws 10mA during measurement, which is only for a fraction of a second. So with all of the sensors connected to the 5V pin of the Arduino and essentially sleeping until they’re called individually for that fraction of a second, you’ll never run the risk of drawing too much current.
Now, if you switch to a different type of sensor, that is something to be cautious about, but it’s not a problem for the 5TM. And yes, connecting the external supply’s ground to the Arduino ground could allow you to power the sensors with the external supply, but it’s totally unnecessary in this case.
The hardest thing about hooking up multiple sensors to one Arduino is connecting all of the wires to one pin. In you case, 6 red sensor wires need to get connected to the digital signal pin, 6 white wires get connected to 5V, and 6 bare wires go to ground. You’ll either have to get creative with a protoboard or breadboard or wirenuts or something similar.