This Stagi mini has ALMOST 2 diatonic (not chromatic) octaves in the key of C. It has C-D-E-F-G-A-B-c-d-e-f-g-a-b plus F#, Bb, c# and f#. So I have almost 2 diatonic octaves in C.
The other possible keys to play in are F, G, and D, though each of those is much more limited (and also only of the diatonic scale).
The key of F goes: F to the a above the higher f (octave plus 2 -- 10 notes)
G is the same -- G to the b above the higher g (octave plus 2 -- 10 notes)
D has a bit more -- D to the a above the higher d (octave plus 4 -- 12 notes)
It's true that almost anything -- of traditional tunes, Morris, etc -- can be played on this, without missing a note...depending on the key, I guess. In a group, too, it's possible to modify the tune to a harmony.
Some 2-part things, though, on a full-sized concertina, seem to fall into place -- but they lose the lower notes of the harmony especially, if tried on the mini EC. To get all the notes, harmony, that I want in Wildwood Flower (for example), I have to play it in the key of F. The lower notes disappear in the key of C, for that one.
Anyway....I noticed at first how limited the 18-button is, but as I look further for good tunes to play on it, I'm noticing that there's a vast amount that will work out.