chromium-50 has 26 neutrons
chromium-52 has 28 neutrons
chromium-53 has 29 neutrons
You subtract the total mass of the isotope (so lets use chromium-50), which, since each proton and neutron weights about 1 atomic mass unit, is the total of protons and neutrons. So subtract the the atomic number (which is the total number of protons chromium will always have, 24) from the mass, 50 - 24 to get a remaining 26, which must be neutrons
