Holy crap, if that number is even remotely accurate then museums have roughly 1 object for every 106 humans that are estimated to have ever lived in the last 192,000 years [1]. Obviously a power law is in effect and a tiny fraction that have ever lived have objects that survive them to this day, and a significant fraction of those objects are animal fossils and other stuff that is unrelated to humans, but it's still a staggering statistics.
I figured there were at most thousands of museums with tens of thousands of objects each, on average.
A very significant fraction will be natural history specimens. A long time ago, I did the London Natural History Museum's MSc course in biodiversity. For one of the modules, in curation, we visited the museum's external storage site, a warehouse complex in southwest London. I barely have the language to describe the scale of the collections there except to say it was on an Indiana Jones 'Raiders of the Lost Ark' level and there were still vast unprocessed archives of material from biological expeditions dating back to the 1800s. There were entire slowly decomposing cetaceans in enormous metal chests, aisles of taxidermy zooming off to infinity, floors and floors of fossils, large to tiny. At the museum's main site the entomology collections alone have over 34 million items.
This collection only covers 73 natural history museums. One example from the article: "The Smithsonian Museum of Natural History alone holds 148,033,146 objects."
Doesn't exhaustive cataloging explain much of this for human artifacts?
A relatively mundane "object" might result in a large number of separately cataloged objects, e.g. every ring and necklace in a jewelry box cataloged separately.
I figured there were at most thousands of museums with tens of thousands of objects each, on average.
[1] https://info.nicic.gov/ces/global/population-demographics/ho...