A 20-button C/C Anglo can only play in a few keys. It only has the "white" notes and F#, which pretty much limits you to C, G, their relative minors Am and Em, and their modal relatives G and D mixolydian and D and A dorian. Mixolydian and dorian scales have a major 6th and minor 7th. The mixolydian has a major 3rd while the dorian has a minor 3rd. Both are used in a lot of Irish tunes.
CODE
C major / G major
C D E F G A B C // G A B C D E F# G
//
A Minor // E minor
A B C D E F G A // E F# G A B C D E
/
G mixolydian // D mixolydian
G A B C D E F G // D E F# G A B C D
//
D dorian // A dorian
D E F G A B C D // A B C D E F# G A