If ∮ _ S → E · d → S = 0 over a surface, then:

The electric flux through a surface is the surface integral of the normal component of the electric field $E$ over that surface. As net flux given by,$\mathop{∮}_{S}\overset{\to}{E} \cdot d\overset{\to}{S} = 0$, therefore total number of flux lines entering the surface must be equal to the number of flux lines leaving it.