According to this thread a 1 gallon bag holds 890 pieces of .45 brass. 1 gallon has ~ 231 cubic inches, so 1 cubic inch equals ~ 3.853 pieces of brass.
Calculate the volume of your can (L*W*H) and then multiply by 3.853. This should give you an approximate count on brass.
I don't have enough to actually fill a 50 cal can, but ran the numbers and figured you could get about 1400 in there. If you shake them to get better density, I can see 1500 being valid.