You would have to check out each NASA region site and look through the results, not the easiest thing to do. Start here
http://www.nasaproracing.com/
then goto each regional page
You can also go to the member profile section and sort by region status and see how many active racers a region has. Basically the Mid Atlantic is the most popular region (since Spec E30 started there). They had 28 racers had VIR last year and you can pretty much count on there being 10 plus cars at an event. The South East is the next biggest and growing rapidly, they also get 10+ car count fields and Road Atlanta is the big draw there. After that you have a number of regions just starting to grow with sort of 5 car counts. California North & South, OH/IN and the North East. Then there are single car counts in Florida,Texas,Midwest,Rocky Mountain and Arizona but I think they all have at least one car.