My breeder had an ultrasound done and they only found 3 pups. She said it isn't always accurate however I would like to know what your personal experience is and the accuracy of the ultrasound is with your previous litters if you don't mind.
Depending on when the ultrasound was done, a bitch who loses a pup they can resorb it, so it won't pass, but it won't be there either. If it is important to get an accurate count, than an x-ray during the last week of pregnancy is probably a better bet. And even then, they can hide.