I was curious about that too... Wikipedia says 4.1 million, but the "About Us" page on nra.org refers to "our nearly three million members". I'm hoping the number's closer to 4.1 million.
ILA’s ability to fight successfully for the rights of America’s law-abiding gun owners directly reflects the support of NRA’s more than 4 million members—a number that has more than tripled since 1978. Copyright 2008
Maybe that “About Us” page is out of date?