I don't play them myself but most of the serious players I know do carry a roll of harps, which I think will cover maybe ABCDEFG and the same number of natural minors.

Most serious singers I know will get upset if you move a song even a semitone from the key that is best for their voice, and quite often moving a song up a semitone will take it out of fingering range on a guitar - and sometimes the same for dropping a semitone: E to Eb for example is a nightmare on the guitar. So ideally you'd carry 24-and I'm not sure if you'd want the other minor as well, and I thought I'd seen other specific scales too.