The correct statement to find the number of neutrons to include in the model of an atom is:
Subtract the atomic number from the mass number.
This is because the atomic number represents the number of protons (which is also equal to the number of electrons in a neutral atom), and the mass number is the sum of protons and neutrons in the nucleus. Thus, the number of neutrons can be calculated with the formula:
Neutrons = Mass number - Atomic number.