According to the Theory of Nikola Tesla's 3-6-9, there are all together 1 to 9 digital root numbers exist. All other higher or lower numbers are the combination of those digital root numbers.
The number 9 produces an interesting pattern:
09+09=18 (1+8=9)
18+09=27 (2+7=9)
27+09=36 (3+6=9)
36+09=45 (4+5=9)
45+09=54 (5+4=9)
54+09=63 (6+3=9)
63+09=72 (7+2=9)
72+09=81 (8+1=9)
81+09=90 (9+0=9)
90+09=99 9+9=18 and 1+8=9
9x9=81 and 8+1=9
You can continue past 100 as well:
99+09=108 (1+0+8=9)
108+9=117 (1+1+7=9)
117+9=126 (1+2+6=9)
126+9=135 (1+3+5=9)
135+9=144 (1+4+4=9)
...
If you really want to go down a Math Rabbit-hole, watch this video:
https://www.youtube.com/watch?v=6ZrO90AI0c8
You can also take 2 aspirin first, the google "Tesla 3 6 9"...
