six missing tones?
according to traditional music theory there are five "missing" tones, the half-steps in the whole steps in a diatonic (do-re-mi) scale. That scale has 7 tones (plus the octave, the second "do") and plus 5 7+5 = 12, the "twelve tone scale" of half steps.
I don't know how one shoehorns in six missing tones. One could do a 13 tone scale within an octave (= frequency doubling) but then all the intervals would be a little smaller and one wouldn't have "do re mi …" any more.