Taking H.R.1022 as the operative definition (which includes the SKS, M1 carbine, Ruger Mini-14, etc.), I'd guesstimate 16-20 million, which is slightly more than the number of licensed hunters in the USA. And that's not even counting non-"AW" guns that have over-10-round capacities.
FWIW, AR-15 sales alone are running over a quarter million a year, and accelerating. It is demonstrably the most common centerfire target rifle and defensive carbine in the country, and so would pass any reasonable common-use test.