In physics, astronomy, and chemistry, number density (symbol: n) is a kind of quantity used to describe the degree of concentration of countable objects (atoms, molecules, dust particles, galaxies, etc.) in the three-dimensional physical space.
Applicable units are those of quantitykind:NumberDensity
http://en.wikipedia.org/wiki/Number_density
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In physics, astronomy, and chemistry, number density (symbol: n) is a kind of quantity used to describe the degree of concentration of countable objects (atoms, molecules, dust particles, galaxies, etc.) in the three-dimensional physical space.
$n = \frac{N}{V}$, where $N$ is the number of particles and $V$ is volume.
http://www.iso.org/iso/catalogue_detail?csnumber=31894