It is hard to say what the real number is although I would wager it is higher than 20%. According to an old gallup poll I found 34% of Americans own firearms while I found a CNN article that reports 50% own a gun (I think this number is a bit high). I was not able to find any data from the ATF or FBI although somebody else may know where to look.
Whatever the number is, it does concern me a bit as I think there is a greater majority who do not own firearms as opposed to those who do. This doesn't look good for the future of the 2A and gun owners.