I keep seeing this claim. 20,000 is such a nice round number. Jon Vernick, director of the Johns Hopkins University Center for Gun Policy and Research, and colleague Lisa Hepburn claimed to have examined gun laws and their actual number and conclude that the figure is essentially a myth. They date the origin of the number to longtime NRA supporter Congressman John Dingell (D-Mich.), who first used the number, without any attribution, in 1965. It's never gone away.
"Twenty Thousand Gun-Control Laws?"
Jon S. Vernick, Johns Hopkins University
Lisa M. Hepburn, Harvard University
http://www.brookings.edu/es/urban/publications/gunbook4.pdf
Evaluating Gun Policy
Effects on Crime and Violence
Jens Ludwig and Philip I. Cook, eds.
Brookings Institution Press/Brookings Metro Series 2003.
http//www.brookings.edu/press/books/evaluatinggunpolicy.htm
"Twenty Thousand Gun-Control Laws?"
Jon S. Vernick, Johns Hopkins University
Lisa M. Hepburn, Harvard University
http://www.brookings.edu/es/urban/publications/gunbook4.pdf
Evaluating Gun Policy
Effects on Crime and Violence
Jens Ludwig and Philip I. Cook, eds.
Brookings Institution Press/Brookings Metro Series 2003.
http//www.brookings.edu/press/books/evaluatinggunpolicy.htm