The Avogadro number of 24g of Magnesium is the same as that of

The molar mass of magnesium is $24 g$. So $24 g$ of Magnesium has $6.02 \times 10^{23}$ magnesium atoms. The oxygen molecule $O_2$ has molar mass $2 \times 16 = 32 g$. So 32 g of oxygen molecules also contains $6.02 \times 10^{23}$ atoms of oxygen molecules.