One.

Wilson, Carl.

Let's Talk (About Love)
(brackets mine)

Pg.77

...Brain science has shown how musical pleasure is structured by expectation and familiarity, in a particular song (when will the pattern resolve, and how?), between songs (is this music like other music we know and like?) and between genres (do you know the rules of this kind of music?). Balancing repetition and novelty is crucial.