I believe that it is more than 15 chapters. I thought it was more like 30 chapters at four year accredited institutions. I could easily be wrong, I couldn't find the info on the website. I know when AST joined the NPC we lost a number of chapters because our chapters were at teaching schools.
In 1972 Iota Alpha Pi quit the NPC rather than be merged or close. They are still around I believe.
http://www.greekchat.com/gcforums/sh...erger+sorority
That is the thread about NPC mergers and closures.
I imagine it would be hard to get in at this point in time.