How to calculate part per million doses

Volume or weight. 1 milligram per 999,999 grams of something else. 1 ML per 999,999 ML. One grain of black sand in 999,999 grains of white sand.

In a solution, there is solute and a solvent.
Now, if the total number of species in the solution is 10^6 (i.e a million) and the number of solute species is 'x', then the concentration of the species in terms of ppm = (number of solute species/number of solution species)*10^6 = 'x' in the above case.