The simple answer is as often as baby wants. Expect baby to feed every 2-3 hours whilst you are waiting for the milk to come in. The interval may be longer in the first 24 hours or shorter- each baby is different. Each time the baby feeds it is stimulating the breast to make milk.
Prolactin is the hormone that stimulates milk production.
The more baby feeds the more milk is produced because you are stimulating the release of prolactin. It will vary slightly each day. But, and here s an old chestnut, if baby is not latched on properly and therefore not able to stimulate and empty your breasts completely, you may become engorged and it may also prevent your breasts from producing the milk in the right quantities. Night feeds are especially important for a good milk supply. There is no need to give extra fluids, so long as baby has free access to the breast, and can have a quick drink when she needs it.