Approximately 1.7 million registered NFA items (includes machine guns) out of an estimated 400 - 700 MILLION guns.
Per a 2016 ATF FOIA response, there are just under 500,000 registered privately-owned full-autos in the US. Only 175,000 are transferrable pre-86 (or pre-68 import).
So we’re talking about a fraction of a percent of privately owned guns.