Also Jossed by Zi-O, which seems to be using 20 as its numerical motif (if anything).
The "II" in Kamen Rider Zi-O II's name can read as "11".
The events leading to the plot happened 10 years ago.
The series is ending with 45 episodes, and 45 is a multiple of 9.
The highest numbered level a Rider can reach is Level 99.
It's Sentai sister series, Uchu Sentai Kyuranger, has 9 rangers on the core team, and uses the number 9 as a gimmick.
Muteki (Emu's final form) has 9 spikes on the helmet.
Most video games usually cap the number of coins you can get at 99 before you get an extra life (and after that, it goes to "00" and resets).
The Rider Gasshat's shape closely resemble a 9.
3 Heroic riders get the main focus, and 9 is a multiple of 3.
Number nine can be read in Japanese as kyu. In Japanese, Kyuukyuu means first aid/rescue.
Most Riders in this series work as doctors, with the main character being an intern at a hospital.
Rider Ghost also has 8 hero spirits in his arsenal before getting the Toucan boost (his own, Musashi, Edison, Robin Hood, Newton, Beethoven, Billy the kid, and Benkei).