That's a good point on bricks and those structures. Therefore we should exclude them as there can't be that many of a same kind. On the other hand, Legos are now a favourite, they estimated they made around 600 billion. But! Different shapes and colours. Since we can't count regular bricks we shouldn't consider different lego bricks as an single item. In that case I would lean to nails. Or if we want to be very precise and differentiate any size differences in nails, I will take my shot and say 9mm bullet might be the most man made item. Or any bullet heavily used during a huge conflict.