How does the number of atoms in one mole of beryllium compare with the number of Atoms in one full of magnesium?

One mole of beryllium contains the same number of atoms as one mole of magnesium, which is Avogadro's number, approximately (6.022 \times 10^{23}) atoms. This is true regardless of the element, as one mole of any substance always contains this fixed number of entities. Therefore, if you have one mole of beryllium and one mole of magnesium, both will contain the same number of atoms.