Dust Particle Size Formula Needed

How i can calculate the number of dust

You need to know the average mass of a particle in mg, you ought to be able to get that based on the density of the material the dust is made from and the particle size.

By dust I am assuming you mean particles of contaminant. However, you are asking for mg/m³. The conversion from ppm to mg/m³ is simple if we are dealing with a known substance/element as opposed to, say, the various components that make up 'dust'

The best that I can offer is this:

Using a particle counter such as a Lasair laser counter or a CNC (condensation nucleus counter) you can determine at a ppm level just how much particulate matter you have per cubic foot or meter of sample as well as the cumulative number of particles of varying sizes. The problem then becomes one of conversion from ppm to your desired measure. So, without knowing the weight of the particulate matter i.e. what the dust is made of, I am stymied.

It is worth noting that I am basing my advice (if I can call it that) on UHP practices as related to process piping in a superclean/ultrapure environment.

cr3 (no relation)

Dont know if this is what you are asking, but if you want to know the amount of dust in, for example, a m3 of air, where the dust is the pollutant, a sampler can be used to process a given volume of the contaminated air through a flat filter paper. The weight of the contaminant can then be obtained via difference from the tare weight of the filter.