ITs just the same as us really - she will produce what they need and it will increase if there are more mouths to feed. To prevent mastitis I think you can only keep a close eye on her teats to look for signs of infection especially redness and heat. Did she wean them very quickly last time? Make sure she is on a good quality growth (e.g. puppy) food to get the extra nutrients that she needs above normal adult food. HTH