What is the mass number of an isotope

The mass number given on the periodic table is a weighted average of all the isotopes. You get the exact mass of an isotope by adding the protons (atomic number) to the number of neutrons in that isotope.

The number of nucleons in the nucleus is the atom's mass number, that being said, each isotope of a given element has a different mass number. so there isn't one single mass number for an isotope.