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Particle number (often symbolized N) is a term for the number of objects (e.g., molecules) under consideration, e.g., in formulas, including simple formulas for calculating number density (n) or column density. The term is used in this manner in chemistry, thermodynamics and astrophysics, for example. The symbol may be subscripted or modified, e.g., NHe for number of helium atoms or NHI (or NHI) for number of neutral atomic hydrogen atoms.
The symbol N and its modifications are sometimes used for number density or column density. These do denote a simple number of particles, so care must be taken to determine whether the symbol is intended to mean an absolute count versus "per unit volume" or "per unit area".
Note that the term particle number is also used with (possibly) a different meaning in health and regarding air pollution, etc. Often it is used to refer to particles of a particular composition and/or size of interest in the air, or in someone's bloodstream, etc.