Last year's rules stated that the best 8 finishes counted towards the championship. I think it was 10 races in 2004.

Currently, there are 12 races on the 2006 schedule, but there are some open weekends where the Roebling 3 hour might be added. That, obviously, brings the total to 13.
7 out of thirteen races (or, again, 1/2 plus 1) sounds very reasonable to me. That way people can easily skip some of those races tightly packed into May and June and still have a valid chance to win.