Why do you have 8 notes on a musical scale?

The standard Western musical scale has 12 notes, not eight. These notes are:

A

A#/Bb

B

C

C#/Db

D

D#/Eb

E

F

F#/Gb

G

G#/Ab

Half notes (sharps and flats) can be added between any of these twelve to further alter a note's tone.