Well, lets look at it pragmatically.
There are an estimated 300 million legal guns in America.
~34% of households have a gun.
There are 116.7 million households.
Average size of a household: 2.58 people.
Using these numbers we get an average of 7.56 guns per gun owning household.
Assuming every person in the household owns an equal share of the gun, we get ~3 guns per person.
Now, I think we can safely assume that not all people in every household with a gun owns guns, but if we say that statistically speaking, the number of gun owners will fall down the middle (between 1 per hh and 2.58 per hh) we can say that the average gun owner will have ~4 guns.