I've read a few other opinions. Some say one teaspoon for 5 galleons. Some say one teaspoon for 10 gallons. Some say it depends on how quickly you want it to be disinfected.
It also depends on how "dirty" the water is. Start with a lesser amount. Maybe 1 teaspoon per 16 gallons as above. Stir and wait a bit, if the water has a slight chlorine smell then you added enough. If you can't smell it at all, add more until you detect a faint odor of chlorine.
If you really want to be sure too can buy test strips to measure the chlorine parts per million.