What does the difference between the mass number and the atomic number tell us?
For example, the most common isotope of fluorine has an atomic number of 9 and a mass number of 19.
The atomic number tells us there are 9 protons in the nucleus (and also 9 electrons in the shells surrounding the nucleus).
The mass number tells us the nucleus contains 19 particles in total. Since 9 of these are protons, the difference, 19-9=10, are neutrons.