How much is the number of neutrons?

As a seasoned individual in the realm of cryptocurrency and finance, I often encounter complex numerical concepts and calculations. However, today's inquiry seems to veer slightly off that path, delving into the realm of atomic structure. So, let me pose the question with a touch of curiosity: "When we discuss the number of neutrons in an atom, what exactly are we referring to? Is it a fixed quantity for a given element, or does it vary? And how does one determine this number precisely? Understanding the makeup of atoms and their constituent particles is fundamental to many scientific disciplines, and I'm eager to grasp the essence of this query.