The total of the masses of protons and neutrons in an atom's nucleus is known as its atomic mass.

Given that the mass of the helium atom is 4 u and two protons are present in its nucleus.
So the number of neutrons will be
Number of neutrons = atomic mass − number of protons
Number of neutrons = 4−2
∴Number of neutrons = 2
Therefore, the helium atom has 2 neutrons