The percentage of U.S. households with guns has been trending downward for years. However, gun-owning households have more firearms than they used to — 8 on average vs 4 in 1994 — so the number of guns in the U.S. has actually gone up, per the Washington Post's analysis of multiple polls.
Methodology: "Researchers surveyed about 1,500 adults in each of the years studied and asked them, 'Do you happen to have in your home (if house: or garage) any guns or revolvers?' Numbers for gun ownership tend to vary across polls, depending on responders' willingness to divulge ownership information and other factors. Still, the downward trend of gun-owning households is consistent across data sets.