If I broke everything down into its components, that figure wouldn't even be close. In fact, I know of so few people who would come close, I'd question where this figure came from. I'm not in the US, btw.
I see you also have too many mugs.
I have that many books plus 500 CDs and 200 vinyl albums. My wife has a couple of hundred handbags and a similar number of coats and jackets.
But books are by far the largest class of thing in my house. I can't imagine having 300k items, even 30k seems rather unlikely in a house where you can see the floor!
Edit: I'm not in the US either so my house is probably smaller than the average US on though possibly larger than the other Brits commenting here; I'm in Norway.
I know the classical exaggerations of these items, but I had no idea someone would actually gather such a collection.
A few hundred handbags and coats? That is an incredibly large number. Could that not be reduced by an order of magnitude?
There aren't that many in actual stores... this can't be right.
The number is clearly an estimate, as getting the real number would be a colossal undertaking for no real benefit.
A rational person does not count the pill bottle as 251 things because it contains 250 pills. But that's the only kind of bloody-mindedness that could ever get you to an "average American home" filled with 300k "things". If you have any LEGO sets, each one of those little one-stud circular pips would have to count as a separate thing. But your dining room table would also be one thing.
We already have cromulent measures for the muchness of one's stuff in terms of kilograms (or pounds) and liters (or cubic feet).
If I plug an old ATA drive into an old chassis, does my thing count decrease by one, because I have the same number of computers but fewer loose hard drives? The volume remains the same, though it is arranged more efficiently. You traded empty space inside the case for empty space outside of it--in theory, you could store baby clothes in there instead of hard drives. The mass also remains constant.
In any case, I would not get that number from an anti-hoarding or de-cluttering consultant. I would objectively measure the change in mass of a moving truck after a household has loaded all their possessions essential enough to move between houses, but not essential enough to keep it under direct supervision. And by assuming that the truck is packed as densely as possible, you can also get a subjective estimate of total volume of possessions.
That would be a really easy way to estimate the median mass of possessions in an American home, without any of the temporary clutter that gets trashed instead of moved. And there would be no thing-equivalence between a dining room chair and a pencil stub, or between a television and one coupon in the junk mail, or between a refrigerator and an ice pack. Mass: x kilograms. Volume: y liters. Numbers where an average actually makes sense.
http://www.ourhappy.space/2015/05/25/count/
tl;dr - The figure seems to come from Regina Lark, a "Professional Organizer" who "specializes in working with people with chronic disorganization, ADHD, and hoarding disorders, as well as folks who have way too much stuff," although she suggests 30,000-300,000 items. I still think 30,000 is way off, however. I'd struggle to get to 3,000.