As I understand it it's the times that you've seen that exact play regardless of opponent.
So in concept if you play in the NC East and every team in your division plays a grinding running game you should build up familiarity against those plays more quickly than others.
Oh, is this true? THis is completely different than I thought then. Is it the number of times the play has been called against my team or the number of times that specific opponent has called that play?