20 percent of the female's weight can be eggs.

Thus, the female herring carries 200 * 0.2 = 40 gms of eggs.

The average egg has a diameter of 1.3 mm.

The volume of each spherical egg is thus (pi/6) *(1.3³) = 1.1 mm³.

Assume the density of the egg is 1 gm / cm³ (it's actually a little larger because the egg contains a large amout of protein that has a density of about 1.3 gm / cm³). Also assume the eggs are tightly packed (there actually must be some protein gel or water in the roe sac because the eggs are fairly rigid and can't pack without some space inbetween them).

The 40 gms of eggs would thus represent about 40 / (1.1 / 1000) = 36,000 eggs. This in the middle of the 20-60 thousand range for fecundity (the number of eggs or live fish per female) given by the reference "UC Berkeley Paper" (URL at end of essay).