There are actually twelve notes in Western music. There are seven notes in a key, the 8th being the octave. Of course, there are actually an infinite number of notes, but not very many that sound good together.
That didn't stop Ornette Coleman.
Yes, but... one can halve the semitones (quarter tones) or even further to microtones, which can be done with synthesizers, electronic keyboards, theremins, fretless stringed instruments (violin, viola, cello, bass, etc.), fretless guitars, fretless basses, pedal steel guitars, slide guitars, trombones, slide trumpets, some valved brass, and the human voice of course.
A lot of the attraction of slight guitar bends, slide guitar, string sections, and vocals, is that they can align the tones closer to the sine wave of the overtone series instead of being constrained by the mathematical alignment of even tempered tuning.