Avogadro's number represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles. We use this in calculating the number of atoms.
2.12 mol C3H8 (3 mol C / 1 mol C3H8) (6.022 x 10^23 atoms / mol) = 3.83 x 10^24 atoms C