That number has got to be waayyyy low. Think about it: How can they make any reasonably accurate estimate? Guns are pretty durable artifacts; they don't just dry up and blow away after X number of years. (I have 120-year-old relics that are still in excellent shape and quite shootable.)
This is why gun confiscation schemes are always so funny. Not only do we not know where they all are, we have no idea how many to look for.