I don't believe there were anything like "millions" of machine guns after WW I. But in terms of per capita, it may have been more than we might think. After all, there weren't a lot of people in this country at the time. There weren't many suppressors either because they were fairly new inventions. The first Maxim silencers weren't even marketed until I think about 1905.
And keep in mind, the cost of guns and ammo were high in that period of time and the average income was pretty low. People of that period were, for the most part, more interested in feeding their families than shooting just to make noise. It's only been in the past few decades that most people in this country have had the disposable income to be able to enjoy collecting and shooting machine guns.