If you know I, ii, iii, IV, V, vi in C then you'll be able to play tunes in A natural minor. Add III to cover A harmonic minor, and II to cover A melodic minor
In real life though the key usually depends on the singer's vocal range, so you won't really get away with learning just one key
In real life though the key usually depends on the singer's vocal range, so you won't really get away with learning just one key