An octave refers to the range from low do to high do (12345671)

Do-re, re-mi, fa-sol, sol-la, la-si are Whole tone.

A whole tone is composed of two semitones, so there are ten semitones above.

There is a semitone between mi-fa and si-do.

So adding these two semitones, there are twelve semitones in one octave! ! !