As others have pointed out, the question cannot be answered without knowing the breaker rating.
That said: In my experience most household well pumps are about 1/2 horsepower. On a 220v circuit they will draw about 7A. Running such a pump on a 15A breaker is fine. And a 15A breaker doesn't need anything like 8 or 10 ga wire.
Because of the long electrical runs, many well pumps are designed to tolerate low voltage. But many people will go to 1 wire size heavier to decrease the voltage drop. Recommended minimums are usually included in the pump installation manual.
Some years ago my friendly neighbourhood pocket gopher chewed through my well powerline. The line wasn't installed to code, being only about 6 inches underground, and running for much of it's length under a rock garden, so rather than try to find the break, I installed a new line. That line was 12 ga. for a 1/2 hp pump that in summer runs 24 hours a day. Pump typically draws 5.7A in use. (Limited by the recharge rate of the well bore)
Note in passing: If this is for a serious duty pump, consider putting in a 3 phase pump with with variable frequency drive. This can reduce pump start/stop cycles, decrease starting currents, decrease energy usage somewhat. Easier on both pump and plumbing.