Only assuming 1000 rounds per owner. I know plenty of people with FAR more than that. Heck, I have an unopened CRATE of Mosin ammo, that’s 880 rounds right there. And an unopened spam can, plus ~200 round in the ammo box.
That’s ONE of my guns, and I’m already pushing 1500 rounds.
The estimated number of guns in the US is in excess of 300 million,
I’ve done some selling of ammo at guns shows. Many people simply have 50 - 200 rounds per in the closet, no more. I’ve sold 2 boxes of 50 for a revolver that someone had just bought, and was going to the range with - how many would they have left after the range? 20 - 50, tops. People like you are not the rule, they’re the exception. Of course, you bump up the average quite a bit.