This subject has, in the past, been beaten into the bushes before. I was going to attempt to find an old thread that covered just about ever square millimeter of the subject, but the archives don't lend themselves to easy search. Or perhaps it was previous to the last datastraphy. I can't remember how long ago, but we had all the math laid out according to the FIA calculations (car type equations), and I believe that the SCCA settles on 25 cars per mile. But even with that, a Chief Steward can consider extending that depending on the track, and conditions. They have that authority according to their sanction's GCR.
HST---It truly is a number that is considered, and applied at the discretion of the sanctioning body, event official, or simply whomever is the over-all authority for presenting/promoting the event.
25-ish per mile is probably the best 'rule of thumb', but some people will have bigger thumbs... like Chump Car.
How about let's try this...
We get 60 entries in the CSCC Enduro, and figure out if that's too many (it shouldn't take too long). Then in March perhaps we'll see that the traffic jam of 55 cars going into 3A after the inevitable restarts during the IRDC Enduro may not be quite enough.
Here's some 2010 FIA stuff. It's a little dry reading in spots, but many questions in regard to the basics of race operations, and event organization are answered well. Check it. You may be amazed at just how easily it may be understood the first time through.
http://www.fia.com/en-GB/sport/regulations/Pages/InternationalSportingCodeA.aspx
Now go skim the ICSCC Regulations.