According to the BATFE...
From 1986-2010...
total Title 1 firearms manufactured = 99,268,206
total Title 1 firearms exported = 6,691,196
total Title 1 firearms imported = 45,259,226
So from those numbers, there are 137,836,236 Title 1 firearms in the USA.
What is not known is how many firearms are in the USA from before 1986.