231 cu. in. to the gallon, and 50 empty .38 special cases take up about 11-12 cu. in. So 2000-2100 rounds, I'd say. (Weight doesn't figure into the calculation of volume, and a wadcutter round takes up no more volume than an empty case.)
This could be maybe 10-15 % low, because in a larger container the cylindrical volumes would have a better chance to settle and get into the densest packed position. Empties in a small cardboard box can't do that as well.