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., $\oint \overrightarrow{B}·d\overrightarrow{s}=0$, 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.