You're recalling a very old guess; an analysis since then https://web.archive.org/web/20190121202815/http://weaponsman... puts the number anywhere from 412 to 660 million.

The old estimates are bad, for example, 275 million new in to the market guns were counted by a system the BATFE introduced in 1999 for high volume manufacturers, importers and wholesalers, a total of 66 FFLs, that no academic had previously used in an estimate.