I found this on a post over at the WC boards. Check it out. This is the attendance numbers ( number of fans attending the races, and type of race )

Indy 500 - 350,000 est. Open Wheel
Brickyard 500 - 320,000 est. NASCAR
LeMans 24hr - 250,000+ Sports Cars
Nurburgring 24 - 194,000 Touring Cars
Sebring 12hr - 180,000 Sports Cars
Daytona 500 - 150,000 NASCAR
USGP F1 - 150,000 Open Wheel
Spa ETCC - 100,000 Touring Cars
Spa 24hr - 90,000 Touring Cars
ETCC Magny Cours - 80,000 Touring Cars
Daytona 24hr - <80,000 ? Sports Cars
JGTC - 46,000 avg. Touring Cars
Brands Hatch BTCC - 40,000 Touring Cars