How many formula units are in 3.40 moles of #"CuSO"_4"#?
The SAME number of molecules are in ANY “mole” of a compound or element. So, you only need to multiply the number of moles given by Avogadro's Number to find how many molecules there are.
If you want to find the number of atoms in the sample, you can then take the calculated number of molecules and multiply it by the number of elements in the compound. In this example the number of copper and sulfur atoms would be the same as the number of copper sulfate molecules, and the number of oxygen atoms would be four times that value.