Seeds per gram (s/g) refers to the actual number of seeds of a given variety per gram. As some seeds are very small, there may be as many as 10,000 s/g, whereas very large seeds like beans may have to be measured abstractly as in 175 seeds per 50g in order to achieve an accurate average.
The term seeds per gram is always used as an approximation, as the relative size and weight of seed may vary by as much as 50%, depending on freshness, dryness, and even ambient humidity. The Canadian Food Inspection Agency uses a commercial standard scale which is available on their web site: www.inspection.gc.ca