A quick internet search will show that the average daily consumption of a chicken is about .3 liters in the cooler months and up to .5 liters per day in warmer months. Remember that there are many factors that influence intake.
To determine the number of birds per waterer, convert the gallons to liters (3.78 liters per gallon) and divide by .3 for cooler months and .5 for warm months. For example, a 5 gallon Quick Clean Bucket Waterer holds 18.9 liters of water. This is sufficient for 63 chickens in cooler weather or 38 in hot weather. It is best to have more water available than what is actually needed.
Another way to think of it is, 7-12 birds per gallon of water. (1 gal = 3.78 liters = 7-12 birds). Make sure to always have clean and fresh water in front of your birds. If your birds run out of water between chore times, they a need a larger or an additional waterer.