The net magnetic flux through any closed surface is:

From Gauss law of magnetism, we know that the net magnetic flux through any closed surface is zero, i.e., $∮ \overset{\to}{B} \cdot d \overset{\to}{s} = 0$, because there are no magnetic monopoles. Here, $B$ is magnetic field and $s$ is surface area. The number of field lines entering the surface is equal to the field lines leaving the surface.