I've never driven on the track myself. And I haven't been for an Open Track day for cars, only the NASA track weekend and the Open Track day for bikes. For the races I saw during the NASA track weekend, there were probably about 10-15 cars on the track. For the bikes, it seemed to just be a few go out when they want a few laps, pit, tweak and return eventually. Never seemed too crowded on the track.
I'm not certain about the times, but I'm friends with the BD & Technology Director from a previous job so I'll try to get some clarification from him tomorrow and let you know.