124 u of ${\mathrm{P}}_4$ will contain :

The atomic mass of P = 31 amu.

Molecular mass of P₄ = 4 × 31 = 124 amu.

Now, 124 g of P₄ contains 1 mole of P₄ = N_A molecules of Phosphorus=4N_A atoms of P(Phosphorus).