It is important to have the correct size drinker for the amount of hens you have.

The average laying bird will drink up to 500 ml of water a day, by taking a sip and then tipping her head back to swallow. She will drink little and often, increasing in regularity if suffering from nutritional deficiency or illness, less if she is not laying. If a hen is deprived of water for 24 hours,it may take up to 24 hours to recover, during which time it is unlikely that she will lay. If the period is over 36 hours, she may go into moult followed by a period of poor laying.   

So some spare capacity will always be helpful, especially if you are unable to refill for any reason or away over night and do not have someone to look after them at short notice. A spare drinker will always be handy for just these events. The following guide is for the minimum water requirements of 3 birds:

Chicks 1-7 days old - 250 ml

Growing birds 1-4 weeks old - 500 ml

Growing birds 4-12 weeks old - 1 litres

Adult birds from 12 weeks old - 1.5 litres