How do we get mass number

The mass number of an element's atom is gotten by adding the number of protons and neutrons of the atom. Electrons have negligible mass compared to the protons and neutrons hence do not matter. The mass of a nuclear particle (protons and the neutrons) is based on the 1/12 mass of carbon which is used as the standard for getting the mass of other elements.

If an element has isotopes the relative mass of the element is gotten by adding all the atomic mass numbers of the isotopes and dividing the sum by the number of isotopes.