There are 7000 grains to the pound. The math should look something like this:
7000 ÷ (the weight of your load) = (the number of loads in a pound of powder)
Example (assuming a load of 35 grains):
7000 ÷ 35 = 200 cartridges from a pound of powder, not counting for spillage or loss.
With black powder, of course, if you are measuring by volume, you would need to weigh some charges to come up with the actual weight in order to have an accurate estimate the number of loads you can get from a pound.
Hopefully that helps.
Griff