I would have 12 ga. wire on a 120v 20 amp circuit that I was going to push hard, and not spindly smaller stuff, and I would aim for that circuit to be dedicated to a single outlet. That's ideal. (Obviously you need 10 ga if you move up to 240v.)
Correction. Wire gauge is based on current. The wire requirements for 240V are exactly the same as 120V.
What else you have on the same circuit matters! You can't run max load on one outlet while another outlet on the same circuit controls, say, the dishwasher... or you plug a vacuum cleaner into it! If you do that, then you may pop the circuit.
That is right and often wiring can be hard to trace down unless you test every outlet in the house. Contractors can be cheap or lazy so there may be more on one branch circuit than what it first appears to be. This is why if someone is looking to pull some serious power (2, 3, 4 KW of load) it really makes sense to run dedicated circuits.
The miners are on the miner circuit and nothing else is on them.
If you've got the ability, I think D&T's suggestion makes sense. Personally, I've got a few solid 120v circuits on new 12 ga copper (which I wired myself!) that don't have a lot of load on them, so I plan to run a couple of miners on them, rather than going to 240v.
If you run out of capacity you can double it by using the same wiring and changing the outlet and breaker to create a 240V circuit.
In US 120V is
hot
neutral
ground
240V is
hot leg A
hot leg B
ground
So same # of conductors you are just using 2 hots instead of 1 hot & 1 neutral. So you can upgrade without redoing the wire run by just changing the outlet and breaker (from single pole = 120V to double pole = 240V).
