“I take it to mean the number of people who own at least one registered gun, “
The vast majority of firarms in the United States are not registered and are untraceable.
https://www.ammoland.com/2018/08/hundreds-of-millions-of-u-s-guns-are-unlicensed-and-untraceable/
True, but listing gun owners doesn’t equal guns owned so in both cases the stat is pointless