You mix the nutrients & water to make a nutrient solution. Then you water the plants normally. There is no set amount that each plant gets, it depends on a lot of things. Water when the pot is light weight and the top couple inches of soil feel dry but not bone dry. Water until you get some runoff from the drain holes at the bottom.
Twelve 5 gallon pots should be getting WAY more water than 2 gallons. I would be giving them closer to a gallon each.