Quick History of the Harmonica
The harmonica was invented some time around 1821 by a 16 year old, German clock maker, Christian Friedrich Ludwig Buschmann. Nearing 1859, Mattias Hohner began producing harmonicas and made approximately 650 of them. By 1879, his company manufactured over 700,000 and it wasn't until the turn of the century that Hohner Harmonicas were really being mass produced - the number reached 5 million harmonicas annually. 1920, the figure had risen to 20 million! That same year, the total output of harmonicas from Germany exceeded 50 million - of these, 22.8 million went to the US, 5.4 million to the UK, 3.1million to India and 1.3 million to Italy. In fact, there were very few countries to which the harmonica was not exported and factories were also being set up in many parts of the world to try to keep up with the demand.1 I couldn't find the numbers of harmonicas made yearly but one company (out of many) has recently reached production numbers in the billions.
Kinds of Harmonicas
Generally speaking, there are 2 types of harmonicas: diatonic and chromatic. Diatonic refers to a selection of notes having no sharps or flats other than those prescribed by the key signature. For example, something that is diatonic to C major has the notes C, D, E, F, G, A and B, but no sharps or flats; something that is diatonic to C# major has the notes C#, D#, E#, F#, G#, A# and B#, but no flats or naturals; and so on. Chromatic is a harmonica that has a full chromatic scale repeated every octave and employs a slide to play some of the notes. Stock chromatic harmonicas normally come with Solo tuned where the full scale in the key of the harmonica is laid out over 4 holes for every octave and by pressing the slide in the pitch of each hole is raised by a semitone. The most common key available for a chrom is key of C, the same as the white keys of a piano.