How about Pennies. I have a jar full of them @ home.
Bristles, Like in toothbrushes, brooms, vacuums, scrub brushes, weather stripping. I like screws to.
Winner!
Has to be the transistor:
60M x 6B = 60 quadrillion transistors produced in 2002 alone. I’m sure the per-year production has gone up since then - and each year, as the newest devices get more complex and powerful, we end up with more and more transistors in service. I may own a house with thousands of nails in it, but altogether the electronics devices I own - car, phone, PC, TV, etc. - contain trillions of transistors.
Would Gravel be man-made? It’s a naturally occurring substance but reshaped and sorted into useful “objects”, athough they are not used individually.
Carbon dioxide molecules.
I thought about it before I opened the thread and my answer was nails. But if you count every one of the millions of transistors in each chip, it’s gotta be transistors.
Digital photo pixels.
…typo? 
Inches of copper wire. Human cells?
Interesting thing about tires, do you know who the worlds largest tire manufacturer by quantity produced? Lego.
skin cells? everyone sheds em constantly. How about shit?
What you see lying around in the outdoors are cigarette butts, plastic water bottles, and shotgun shells.
Locally, I would say hippies, “travelers,” vagrants and bums. Here, I would say zombie threads.
Except OP stipulates that we shouldn’t count those that have been discarded. That cuts down the count quite a bit. But even so, lots and lots.
Having been primed by a few other posts, I read this as “penises”. Which would be… disturbing.
Grains of flour,salt or sugar.
Excuses.
Chopsticks (to give a different cultural slant). Include all the disposable ones, and it must be a lot.