ReciprocalVolume?

[chucker]

Is your feature request related to a problem? Please describe.
I have a particulate matter sensor that gives measured values in µg/m^3 (so I can use Density), but then also measured counts in #/m^3.

Wikipedia seems to call this https://en.wikipedia.org/wiki/Number_density.

I've also found the ReciprocalArea type, which is pretty close, except I need ReciprocalVolume.

Describe the solution you'd like
NumberDensity and/or ReciprocalVolume types would be nice, with factory methods such as FromInverseCubicMeter.

[chucker]

(NumberConcentration might also be the correct name here. I'm not sure.)

[ricardoboss]

See also: https://en.wikipedia.org/wiki/Concentration#Number_concentration

[angularsen]

This feels a tiny bit fringe/exotic, but I'll accept it.

From googling a bit, it seems number density is maybe the more commonly used term.
I have no hard preference, but reciprocal volume seems the least common of them.

I'd suggest NumberDensity, alternatively NumberConcentration.
If you want to go ahead with a pull request, steps are outlined here: https://github.com/angularsen/UnitsNet/wiki/Adding-a-New-Unit

I'm happy to assist.