could there only be 1?? Just wait and see what she brings.
Vets are very often inaccurate, if there is one then there could be more, pups are great at hiding and vets notorious for not finding them.
Just get all your whelping preparations ready and see how things go, be prepared for anything which is what most of us who breed do anyway.

Nothing is ever set in stone, so keep a close eye on her after the first is born.
We shall see in approx 5 days what happens, keep us informed very interested to see how many pups there actually are.
Good luck and any probs or worries someone will always be here.