"Number of Particles", also known as the particle number, of a thermodynamic system, conventionally indicated with the letter N, is the number of constituent particles in that system.
Applicable units are those of quantitykind:NumberOfParticles
http://en.wikipedia.org/wiki/Particle_number
"Number of Particles", also known as the particle number, of a thermodynamic system, conventionally indicated with the letter N, is the number of constituent particles in that system.
http://www.iso.org/iso/catalogue_detail?csnumber=31894