If you look up "list or rampage killers" on Wikipedia, it gives you a list you can sort through. They define a rampage killing (or a spree killing) as essentially one with at least six dead victims (not including the perpetrator), or with at least ten dead plus injured. With that, you get a number of 20 in the United States since 1982.

However, that's only if you don't count vehicular manslaughter rampages, which would raise the number to 24, but that doesn't really factor into a discussion on gun control. That also leaves out intentional arsons resulting in mass casualties (two since 1982), intentional plane crashes (one), and terrorist bombings committed by American citizens, such as Timothy McVeigh and Eric Rudolph.

But it's entirely true that other countries have their share of rampage killings. It's not really an American phenomenon, it's just that American events get the press coverage. Personally I think that the overall level of gun-related violence is a better argument for more gun regulations than any killing spree incidents. People who truly want to kill a lot of other people are the outliers. Much more damage is caused by everyday gun violence, and I wouldn't be surprised if more people shoot themselves or someone else accidentally every year than have been killed in all the gun-related rampages in US history combined (but I don't have a number on that).