Perhaps one of the most common questions I have encountered over the years as a member here and as a friend helping other mothers in "real life" relates to supply. I believe it is the single biggest factor in a woman throwing in the towel early on in the nursing experience.
What many mothers do not seem to understand is that breast milk is extremely digestible and that an infant's stomach is literally barely bigger than a walnut in the beginning. Put these two things together and it equals a baby who eats very frequently. It is not unusual for a typically developing newborn to nurse every 1-2 hours during the day and 1-3 hours at night.
Growth spurts are another point of contention with nursing moms. Babies demand a 24/7 dining experience. They require constant access to the breast and will frequently eat hourly during these periods of time. My boys would literally nurse almost non-stop for 30 minutes to an hour, snooze for 20-30 minutes and start nursing again. During the very early spurts, they would nurse even more frequently! That is normal and is okay. You are still producing even if you feel "empty" Try varying your nursing position to tap into more milk. If you usually do the cross cradle, try the football or side-lying. Also one of the best things for a nursing mom trying to keep up with the demands of a nursing baby is to take a baby holiday in bed. Sounds more exotic than it is. Take your wee one and get away from the demands of the house and sequester yourself in your bedroom. Take that time to bond and nurse without the distractions. I know how difficult that can be with more than one child, but if you can make the time, it will make for a more relaxed mom and baby if you can reconnect without all the stress.
And your baby may fuss, what was once a flowing source of milk is being taxed during the first day or so of a growth spurt. But rest assured, they are getting enough if you are seeing plenty of wet diapers (6-8 a day once they are 1-2 weeks old, during the first 4 days or so they should have one wet diaper per day of life). Once your supply catches up, it will be smooth sailing and then your baby may be very sleepy for a day or two (this is the actual growth spurt) You will see very frequent growth spurts during the first 4-6 months, coming about every 2 weeks for the first 2.5 mos, then spreading out a bit.
Perhaps the biggest mistake a mom makes right now is panicing and offering a supplemental bottle. For every ounce you give of a supplement, that is at least one ounce you are not producing. Your breasts work on supply and demand, if the demand is not there, the supply won't be either. Many than create a self-fullfilling prophecy, they doubt their supply when it is normal, add supplementation, then kill the supply. Patience is key with nursing. It is not an easy thing to do, but it is so very worth it in the end.
The first 3 or 4 months nursing exclusively can be very stressful, even in the most ideal of circumstances. Have faith in your supply and have faith in your baby to do what nature intended.