For one to five birds, one nipples.
For five to thirty birds, two nipples.
One per fifteen or less after thirty.
Chicks less than ten days old, double the number of chicks per nipple above.
More than three nipples on the bottom of a 5 gallon pail is more than can really be used by the chickens due to space and their resistance to the idea of standing under something where their view is blocked. Remember that chickens are a prey speices and they are always more comfortable with a verticle avenue of escape.
Never let the possibility of a nipple becoming clogged enter into how many nipples you have for a group of birds. Actually underutilization of nipples can be more of a problem. Constant usage allows solid particles in the water more chances to flow out with the water streem and not block the nipple stem open. If you are going to have problems it will be with over flow.