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 also equals the number of electrons in a neutral atom), and the mass number represents the total number of protons and neutrons. Therefore, the number of neutrons can be calculated using the formula:
Number of neutrons = Mass number - Atomic number.