The correct statement should be: The number of neutrons in an atom is equal to the atomic mass minus the atomic number.
The atomic number means the number of protons in the atom, meanwhile atomic mass is the sum of protons and neutrons of that atom (not electrons because they're too light).
So, in order to find the number of neutrons in an atom, we should minus the sum of protons and neutrons by the number of protons. Therefore the statement is false.
