12 plants outside and five pounds sounds correct to me and a little on the upper side.
LOW BALL for outdoors, for starting in May.
You should get at least 10lbs from 12 if they got full sunlight all day.
It depends on how much you tend to them soil water etc... Last few years just midgrade seeds I averaged half pound per plant, no fertilizers just a hole a seed the sand from the hole mixed with topsoil water one time and let nature go. Got rid of males of course but that's it. With good plants and good care over a pound a plant should be average. With a hieght of 5 to 7 feet depending on the strain of plant of course.