Well, each team has 53 players on the roster. Plus they get a five man Practice Squad. I think they still get six if one is a QB, but I'll use five. So...58 x 32 = 1856. And 7 / 1856 = .0037 (approximately). Peter King says "at least seven" so we could round that up to .004 perhaps.
Oh, thou clear spirit, of thy fire thou madest me, and like a true child of fire, I breathe it back to thee.