IMO the number of classes does have an effect on entries. The more classes that get forced into club racing from top down, the more it decreases in-class completion by separating the current driver supply into smaller groups.

I have a GTL car. I am not interested in Majors/Runoffs participation, so I raced by myself at Regionals. Not much fun, so in the middle of the 2014 season I bought an IT7 car after a couple guest drives in 2013.

Why did I leave GT for IT, simple, competition. And that competition brought me out to the track more in 2014 than 2012 & 2013 combined.

If you do add classes (IT7 is a great example) let it come from the bottom up. If it grows great, if not kick it to the side (ie the now defunct, Classic GT).

Solid competition increases entries, participation trophies don't.