The mass number of an atom is the sum of protons and neutrons. The number of protons is an atom's atomic number and doesn't vary for a particular element. The number of neutrons can vary. When atoms with different numbers of neutrons exist for an element, they're called isotopes. To find the number of neutrons in an atom of a particular isotope you subtract the atom's atomic number from its mass number.
Here's an example using lead, with has four stable isotopes and an atomic number of 82:
The notation "lead-204" means the isotope of lead that has a mass number of 204. Since lead has 82 protons, the number of neutrons in an atom of lead-204 is 204-82=122. Similarly, lead-206 has 124 neutrons, lead-207 has 125 neutrons and lead-208 has 126 neutrons.