Any of our local club vendors will get you exactly what you need, but just in case you're interested:
Section 3.7 of the rules cover numbers:
Link to doc with rule 3.7:
https://dk1xgl0d43mu1.cloudfront.net/us ... 1499879807
Link to all SCCA autocross rules (just in case you want to really geek out!):
https://www.scca.com/pages/solo-cars-and-rules
3.7 VEHICLE IDENTIFICATION
A. All vehicles must display numbers and class letters on both sides, which must be readable by Timing & Scoring, Course, and Grid workers at all times.
B. Only one set of numbers and class letters may be visible while the vehicle is running.
C. Class shall be represented by the upper-case abbreviated form rather than be spelled out. Ladies’ classes shall be indicated by the letter “L” following the class letters. (Example: “BSPL” instead of “B Street Prepared Ladies”).
D. Numbers and class letters should be positioned next to each other. All letters and numbers must be on body panels, not on windows. All numbers
and class letters must use the same typeface and the same color, and this color must provide adequate contrast to the background color (see Appendix F for examples).
E. Numbers must be a minimum of 8” (20.5 cm) high with a 1¼” (3.25 cm) stroke. Class letters must be a minimum of 4” (10.25 cm) high with a ¾” (2 cm) stroke. In all cases, the height of the class letters must be between 25% and 75% of the height of the numbers. Stroke width must be at least 10% of the height (see Appendix F.)