Considering all the music from all differents cultures on earth, how many notes do we get in an octave? I'd guess something around 24 instead of the 12 of western music?
The answer is, actually, an infinite number of pitches.
When we think about something like computer-generated music, and glissandi within that repertoire (e.g., the THX intro), we see that there is an infinite number of frequencies that exist within the boundary of a single octave.
The answer is infinite
Consider just one note, not even an octave.
Ask 12 violinists to play one note. none of them will hit the exact same pitch, yet the listener would consider that to all intents & purposes, they did.
Even take 12 guitarists.
They all tune by ear to a commonly accepted pitch, then each play the same note on, say, the 3rd fret.
That increases the variance, because not only did they not all initially tune to mathematically the same pitch, they then all pressed with slightly different pressure on the 3rd fret, each stretching the string slightly differently.
Additionally, strings change pitch depending on how hard they are struck or bowed - adding even more variance to the result. So each pitch changes over time.
…and that's when everybody in one band, from one culture & tuning standard, tries to play one single note.
Multiply that by the number of tuning standards there are & how accurately each musician can hit that standard, at each & every note they play.
If you really want to test the human ear's response to microtonality, start with a synthesizer playing simple sine waves. Start with 2 waves & detune in 1/10th cent intervals. Listen what happens.
I can't even begin on the history of tuning standards - let WIkipedia have a go - Equal temperament