The 12 positions don’t refer to the number of holes on a blues harp. They refer to the 12 tones that are used as the basis of most of the music played in the western world (and “No” I do not mean western as in country and western). Start with any note – C or Bb or E or whatever – and there are 12 tones from that starting note up to that same note an octave higher.
Example: C C# D D# E F F# G G# A A# B. Each position for playing a blues harp refers to starting a scale using one of these 12 notes as the root (“starting” note) of the scale. 1st position is the scale that begins with the root note at hole 1 blow (i.e. the key of the harp). The circle of 5ths can help you for the starting note for any position on any blues harp.
– Slim 