Re: not to be pedantic, but..
As near as I can work out, the density of air is about 2.4 x 10e19 atoms per cubic centimeter. For comparison, an example of density in space might be that of the solar wind, at perhaps 3-6 ions per cubic centimeter (compare to an estimated "average" density of the interstellar medium of 1 atom / cc). So solar wind, and indeed the interstellar medium, are substances --- just highly rarefied ones.
Talking about Mach numbers in a plasma makes as much sense as talking about temperature of a plasma (a measure of the kinetic energy of the atoms in a material).
Biting the hand that feeds IT
