Bulldoggie,
In many cases, the numbers are distributed on a first come, first served basis. WFC for instance, the number that appears in the upper right corner of your tech card is your race number. At one of my local tracks, the gentleman at tech inspection asked what number I wanted. I told him I didn't have a preference, so he gave me 330, I guess because my engine is 330 ci.(?)
I hope that was the jist of your question.
Jake...