Placentas are expelled from the mother dog after the delivery of puppy. Delivery of each puppy is followed by the delivery of placenta. Total number of placentas depends upon size of litter. Mother dogs usually eat these placentas because these are nutritious. So, placentas are removed from the dogs within 15 minutes to 2 hours. If placentas retained in the uterus then infection can results. Such condition require vet visit immediately.
There should be a placenta delivered for each puppy born. This is usually after each puppy but not always. The important thing is that they are all delivered. If they are not they can cause an infection that will make your girl seriously ill. If your dog has not delivered a placentas for each puppy, your girl seems ill or depressed, running a fever, lethargic, something just seems wrong, or has a smell or a pus looking discharge after birth you should call/visit your vet. Eating a couple is okay and actually pretty good for your girl but too many can cause diarrhea :-).