The periodic table lists two main numbers for each element. the atomic number is an integer that equals the number of protons. the number of neutrons is not given in the periodic table because it will vary with different isotopes. the mass number of an element is the sum of the protons and neutrons. to find the number of neutrons, the atomic number must be subtracted from the mass number.

Given that isotopes do exist, the mass numbers given on the periodic table reflect the average of the most common isotopes existing. This is calculated:
Atomic Mass=(Fractional abundance of isotope 1*mass of isotope 1)+(Fractional abundance of isotope 2*mass of isotope 2)... depending on how many isotopes there are. But, generally, you can find the most likely number of neutrons by subtracting the atomic number (protons) from the atomic mass(protons+neutrons).