The given atomic number, #Z=7#, was superfluous. All nitrogen atoms have #Z=7#; the presence of 7 massive, positively charged nucleons characterizes the element as nitrogen. But your question asked how many neutrons this isotope contained. And the nucleus can also contain various numbers of neutrons, which are massive, nuclear particles of zero charge. The protons and neutrons together engage in the strong nuclear force, which, at short, nuclear ranges, is strong enuff to overcome the electrostatic repulsion that exists between the charged particles.
And, to labour the point, we have #""^14N#, 7 protons, and 7 neutrons. How many neutrons does #""^15N# have? And how many electrons does each neutral nitrogen isotope have?
