There should be five peaks. In the process of bombarding the Br2 molecule with electrons to charge it, the molecule can fragment into 2 Br atoms. We should therefore expect to see peaks at 79 and 81 m/z. Then we should observe three more peaks from Br2 molecules consisting of the three distinct combinations of the Br isotopes that could be present in the molecule; a peak at 158 m/z where both atoms are Br-79, one at 160 m/z with one atom of each isotope and a last one at 162 m/z with both atoms the Br-81 isotope.