Let us try to learn some basic fundamentals of Numerology-
Ruling Number/Personality Number

This number is the indication of a person’s view point or perspective. This number is just the number of the day of your birth. To calculate ruling number, just add the numbers of your date of birth. E.g. ABC's birthday is on 13th May 1972. To calculate ABC's ruling number add 1+3+5(month)+1+9+7+2 = 28,
Further going 2+8=10
1+0=1
i.e. 1 is the person’s ruling number.
This number projects the person’s innate individuality which no one can change.
See another example,
XYZ was born in December 21st 1980 (12.21.1980)
1+2+2+1+1+9+8+0 = 24
24 = 2+4 = 6 i.e the Ruling Planet Number