Why chromatic scale has 12 notes?

Answer 1

There are twelve notes in an octave:
A-A#-B-C-C#-D-D#-E-F-F#-G-G#

A scale is sometimes determined by the number of notes per octave it uses:
A pentatonic scale has five notes per octave (penta=5)
An octatonic scale has eight notes per octave (octa=8).

The chromatic scale has every note in the octave, hence it has twelve notes.

Answer 2

every scale has 12 notes