My sister breeds Chihuahuas, so I've learned a bit about dog breeding from her. Dogs being bred for the first time will often have smaller litters than dogs that have been bred a few times (this goes for many animals, actually). Also, (and I hope this isn't TMI) the number of puppies a dog has can be dependent on the number of times she breeds the male. So if a dog only breeds with him once, it may result in fewer puppies than if they breed several times. Or possibly even just one pup (though breeders try to avoid this as the puppy can grow too large and cause complications at birth).
Also (and this is a completely unrelated, but interesting fact), dogs can have puppies with more than one sire. This actually happened to my sister as she *thought* one of her males was too young to breed and had left him in with the girls, but he wasn't! She ended up with three puppies. Two sired by him and the third by the dog that was *supposed* to be the daddy. (They did a DNA test when the puppies were born with short hair instead of long.)
Hope that helps!
Rue