follow up - the usual estimate is something like 1/4# of feed per bird per day.  Obviously, the size and activity levels of your birds will affect that rule of thumb, but its a good place to start.  Then watch their behaviors.  If they eat all the food in 5 min and are begging for more, they need more feed.  If they eat for 15 minutes, then walk away with a big pile of food left behind, cut their rations.
With 12 birds, you are looking at 3#/day as a start, your 50# bag should last a bit more than two weeks.
Hope that helps, apologies for the fast response above - it was meant as a place holder, not snark.