The first 12 (or arguably, 11 since 11 and 12 are the same person) riders are of the Showa era spanning from the 70s-80s. Riders 13-15 are from the 90s. Riders 16+ are of the Heisei era in the 2000s.
After the initial seventeen who were more exclusive Kamen Riders in their respective series, a group of thirteen riders was introduced in the February 2002 series Kamen Rider Ryuki (later adapted as Kamen Rider: Dragon Knight), bringing the total to 30:
After Ryuki, the January 2003 series Kamen Rider 555 went back to having fewer Riders, introducing Riders 31-33:
The August 2003 Faiz Paradise Lost film introduced Riders 34 & 35
The January 2004 series Kamen Rider Blade introduces four new Riders, 36-39.