Truth be told, no one really knows. Every HS shell is a blend of all different sizes of shot (4s to 9s) on a more or less bell curve that hopefully averages somewhere close to the nominal number for the size shot printed on the box. You could cut open and count 10 HS shells and not come out with the same number in any shell. But Skfooter is correct, the nominal number for 2.25 oz of 12 gm/cc 7s is 625.