Helium atom has two neutrons. The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus.
Since helium atom has two protons, mass contributed by the two protons is (2 × 1) u = 2 u.
Then, the remaining mass (4 − 2) u = 2 u is contributed by 2u /1u = 2 neutrons.
