And here's how I've done it back then:
1.) open DATA.twt with the texteditor and scroll through it
2.) you should find something like this:
3.) Edit the value of "Default number of opponents in first race" up to 21 for example and the value of "Default number of opponents in last race" to 27// DEFAULT PARAMETERS5 // Default number of opponents in first race
11 // Default number of opponents in last race
0.5,3 // Softness,hardest rank of opponents in first race
3,6 // Softness,hardest rank of opponents in last race
0.8 // Opponent nastiness influencer for first race
5 // Opponent nastiness influencer for last race
4.) Save the file and congrats: you now play C2 with 21 AI drivers on one track which can be even more chaotic! (EDIT: You can also choose another value if that's too chaotic for you!)
I'd recommend to download and import cars because otherwise you have the same car more than one time on a track
BTW: you can also change the values of the powerups in that file, e.g. remove the time limit for the electro ray to make it an permanent powerup