My dog had 12 puppies. How do I know if they are all getting fed? Can she feed that many?

Most large breeds of dogs can feed 12 puppies quite well. But you will probably need to help the mother along by giving her extra food as long as the puppies are nursing. Most dams will need two to three times as much food as they consumed before becoming pregnant.
As a general rule, feed 150% of the maintenance energy requirement during the first week of lactation, 200% of maintenance during the second week, and 300% of maintenance the third and fourth week until weaning. Peak lactation occurs during the third to fourth week. Feed a nutrient-dense food such as a puppy formula or a performance dog diet, and either free feed or give her several meals per day. Do not give her supplements because a good quality dog food will have the exact mix that is needed; plus, supplements can be detrimental to the health of the dam and the puppies. Be sure to change her food over slowly so that she doesn’t experience gastrointestinal distress as a result of a quick change. Access to plenty of clean water is absolutely necessary so she can turn that food into milk for the puppies.
With a very large litter, it is important to keep a close eye on the puppies to make sure they are all getting fed. Use a small scale, such as a postal scale, to weigh the puppies each day. Mark each puppy individually so you can keep track of its weight. Many breeders assign each puppy a color, which can be indicated by special newborn collars, colored yarn around their necks, or nontoxic markers used somewhere on the coat. Some breeders prefer the base of the tail; others mark the back of the neck or a paw if it is the only light part of the pup. You will probably need to remark the puppies every couple of days. If you use collars, be sure to check them daily because puppies grow very fast.
It may also help to place half of the litter in a basket (a laundry basket lined with fresh clean towels works well) placed close to the mother. Let her nurse half of the litter and then, when the puppies are satisfied (they will stop nursing and fall asleep), place them into the basket and put the other half of the litter on the mother. Some new mothers are nervous about this at first, but in time the mom will come to accept this, especially if she can see the puppies in the basket and if they are sleeping. Keep the puppies warm at all times. They will need to feed every two hours. At night, every four hours is usually sufficient.
Moist puppy food and/or yogurt can be introduced to the puppies off of a finger as early as two weeks and on a plate in their third or the fourth week of life. By the end of the third week, the mother may not be able to produce enough milk to meet all the needs of the puppy so additional food needs to be available by this time. By five weeks, most puppies should be feeding on puppy food well and the dam will be nursing less often.
If you find any of the puppies are not gaining weight or appear dehydrated, you will either need to find another nursing mother or supplement with bottle or tube feeding. Consult your veterinarian for more information or assistance with feeding puppies in the first two weeks of life.