Copper has a molar mass of #63.55*g*mol^-1#. What does this mean? It means that #N_A#, #"Avogadro's number"#, i.e. #6.022xx10^23# 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*kgxx10^3*g*kg^-1xx6.022xx10^23*"copper atoms"*mol^-1)/(63.55*g*mol^-1)#
#="an absurdly large number of copper atoms......"#
