That are the ones they know about! It wasn't until recently the feds started rapid background checks. There was a time you could order one from a Sears or Wards catalog!
because of that there could be almost Twice that many out there!
That was before 1968. I remember it quite well.
The estimate of the number of privately owned guns is not from background checks. It is from the number of guns manufactured in the USA, plus imports, minus exports. Those numbers have been reported since 1946, with reasonable estimates going back to 1900.
Privately made guns are not included. Guns sold as surplus from government stocks are not included. Guns smuggled into the USA are not included, nor are guns smuggled out of the USA subtracted.