Use this guide to help you out on how to class your car.
Vehicle Identification Guidelines (PDF)
Also keep these things in mind:
1) You should use numbers from 0-99 for single driver cars and 100-199 for two driver cars. This is in-line with National level numbering shemes. A two driver car is one that has both drivers running the same car in a single heat. A car being driven by two drivers in separate heats is not a "two driver" car.
2) If you're new (and even if you're not), it's best to get individual numbers and letters for your number / class. You can readily invert these to change your number, which could be useful at times.
3) If you've done #2 above, then you should also buy a "1", so you can easily change your number to 1XX for two-driver cars, or 1X or X1. You get the picture.
4) Take the size and contrast considerations outlined in the PDF document above seriously. It'll make life easier on everybody at the event if you do.
5) You number/class designation must be on your car prior to entering grid. Grid is the place where you stay lined up with the other cars during your runs. Having proper identification on your car prior to entering grid makes it easier on the grid workers to organize the heat.
6) Your year-end points (standings) are calculated based on your name and class, not your number. Changing only your number at different events will not affect how your points get accumulated.