The general idea is that your body actually prioritizes your milk-making over its other needs - so if you have a lousy diet then you will feel worse than if you ate well, but you will still made adequate milk for your babies. Supply for babies is mostly dictated by sucking now.
The extra calorie requirements are typically not huge - but they seem to vary a lot from person to person. I was feeding one baby and eating lots of food + some junk, and still losing weight. As long as you are not losing weight too quickly you should be fine.