has to be number 9
did you know if you take a two-digit number like 21, add the two digits (2+1=3),
and subtract the answer from the original number (21-3=18), the final answer is always a multiple of 9
also,
did you know, any number multiplied by 9, the sum of the digits of the answer will be equal to 9.
For example:
6 times 9 equals 54,
5 + 4 = 9
628 times 9 equals 5,652
5+6+5+2=18
1+8=9
Et cetera
number 9 owns all you other numbers