The mind perceives pitch to be continuously variable - there are no "quanta" of pitch - but in music, out of the infinite possible pitches that could be chosen from the pitch continuum, only a limited number are used.
Usually each octave is subdivided into a small number of steps and each of these is repeated in every octave. In almost all musical cultures notes separated by an octave are regarded as somehow equivalent, so that, for the sake of consistency and simplicity, divisions in any one octave are repeated in all others. Scales in which different notes are used in different octaves, or where octaves are not found at all, are rare.
By choosing a limited number of notes the ear is given a structure that is simple enough to be understandable and whose notes are spaced apart enough to be easily heard as different. Ideally within any octave, each note is perceived to be fundamentally different from every other note - each note has a unique identity. When that identity is unique enough it allows for each note's pitch to be varied with vibrato and other decorative techniques without losing its identity and becoming confused with other notes.
The pitches used in purely melodic musics - such as classical Arabic and Indian are generally more flexible and complex than those used in the tonal harmonic music of common practice classical and popular music. Within any one scale we will often find more than seven notes and the distance between consecutive steps can be very small.
But we also find in melodic music the frequent use of both the pentatonic scale - Celtic and Asian songs, and the seven note diatonic scale - Native American and African songs. These common scales often sound very different to how they do in tonal harmonic music, through generic forms of decoration and pitch variation.
On this site I will be examining in detail only those scales which are suitable for use in tonal harmonic music.