Sorry, can't tell you about 9mm BUT I once bought 2 5-gallon buckets of .223 from a guy. I used a postal meter, counted 200 cartridges and weighed that, then on a bathroom scale weighed myself holding a bucket, determined the weight of the bucket with brass and without and determined each bucket filled to the top could hold almost 50 pounds of .223 brass = ~3,600 cases.
Best I can do.