In music theory class we learnt that there is 12 notes notes in music and that there is 7 sharps and 7 flats. And that you need 7 to make a scale. A tone is 2 semi tones and the black keys on a piano is what you would call a semitone. A lot of musicians use a technique called the circle of 5th’s because you can use 3 chords used in a 123 order and make a whole song based of it because you have the minor symbolizing chords as well. When you use the normal numerals its because its the order of scale so say you use the C-Major scale C would be 1. 2d 3e 4f 5g a6 e7 and then c7 to repeat the scale(The last note is always the same as the 1st note to complete a scale)