Copper has a molar mass of 63.55⋅g⋅mol−1. What does this mean? It means that NA, Avogadro's number, i.e. 6.022×1023 individual copper atoms have a mass of 63.55⋅g. And if you can get your head around this, that the mole is simply a NUMBER, it will save you a lot of angst. This is the principle of chemical equivalence: a mass represents a numerical quantity of atoms or molecules. And we can use masses to determine stoichiometric equivalence in chemical equations.
And so here, we gots an 860⋅kg mass of copper......and so we take the product.....
860⋅kg×103⋅g⋅kg−1×6.022×1023⋅copper atoms⋅mol−163.55⋅g⋅mol−1
=an absurdly large number of copper atoms......
