Can I use 12 AWG for a 20-amp breaker?

Twelve AWG copper is exactly what a 20 amp breaker calls for. NEC 240.4(D) sets it as the minimum, and 12 gauge with 20 amps is the standard pairing for kitchen, bathroom, laundry, and garage circuits.

Ten gauge on a 20 amp breaker is also legal, just thicker than required, and worth the money on runs past 75 to 100 feet where voltage drop starts to bite.

Fourteen gauge is the direction you cannot go; it caps at 15 amps.

For aluminum, the answer changes: 12 AWG aluminum is only good for 15 amps, and 10 AWG aluminum is the 20 amp minimum. The metal matters as much as the number.

The 20 amp breaker wire size calculator covers copper, aluminum, and the long-run upsize point.