An octave consists of 12 semitones.
This applies to the Western music system, which divides the octave into 12 equal parts, known as semitones or half steps. In this system, moving from one note to the next adjacent note on a keyboard, for instance, represents a change of one semitone. When you move up or down 12 semitones, you return to the same note name, albeit at a higher or lower pitch, thereby completing an octave.
For example, starting from C, if you move up 12 semitones, you would reach the next C note, one octave higher. This division is fundamental in music theory and is important for understanding scales, chords, and overall musical structure.