I expect many form the Bay Area. My best guess is 30-40 cars from Northern California. The Thunderhill weekend is fantastic and well worth the drive just to watch. It is a great opportunity to experience the performance of a Viper.
The track side vendors are great and provide parts(brake pads, wheels, smooth tubes, rear ends, transmissions, restraints, helmets, fire suits and gloves, tires slicks and street, etc ) and installation for same. Changes for all fluids, suspension set-up help, etc. Instructors for all who want to improve their skill.
Viper Days is the best open track event(driving school and wheel to wheel racing) offered at any price.