Mark,
Some drivers/teams change frequencies week to week. Some use one frequency for practice and switch to another for the race. Some drivers/teams have been known to change frequencies mid-race. Most teams use two frequencies. It would be safe to program both. If you want to listen to all the action you should have a scanner that has no less than 200 channels. If you want to listen to only a few teams, most scanners that cover the 450-470 mhz band will do just fine.
You can pick up frequency lists at the track. Frequency Fan Club and Racing Electronics have the most accurate and up to date frequencies. Through FFC you can also program your scanner for the track at home with your computer and the proper software for your scanner. They cover all the major racing series.
I hope this helps.
Duke