The body makes plenty of tears. If you suddenly find yourself crying over some sad or happy time in your life, you don't suddenly worry that you won't have enough tears left over for the funeral.
Milk production works much the same.
When Emily Fischer decided to breastfeed her first child, the idea of nursing whenever her little bundle of joy demanded made her more than a bit apprehensive. But she was pleasantly surprised when she and her newborn daughter, Lyndsay, settled into a pretty regular nursing pattern once they arrived home from the hospital. That was about as often as she seemed to want to be fed. Sometimes, Lyndsay would even sleep for five hours between feedings. But, as soon as Lyndsay turned about three weeks old, the predictability of her feeding routine went haywire.