there's 12 notes in our western system (c c# d d# e f f# g g# a a# b) and pretty much all the 'natural' scales we know usually have 7 notes (modes, major, minor...) of course, I know there are the pentatonics (5 notes) and the Be bop scales (usually 8 or 9 notes) The chromatic scale has the 12 notes.
What I meant was that at any given time when you improvise, there's at least 7 notes than can sound good out of the 12 that exist. Of course, it's not always true, and it's sometimes more than 7... It's just a little humor!!