How far along is she? You should get her to a vet to do an ultrasound.
You will need a whelping box with safety rails to make sure she doesn't accidentally smother the pups. How many pups does a first litter usually produce?
It depends. Dobe litters can range from 1 to over 10, though about 6-8 is common iirc. You should really have looked this up BEFORE breeding. How much do they usually sell for?
Less than they cost to produce if you do it to benefit the breed.
Remember that once these pups are born, YOU are responsible for them for the rest of their lives. Carefully screen your puppy buyers and make sure that if they for any reason ever can't keep the dog that you are able to take it back. Also, get multiple forms of contact information - it's easier to lose touch than you might think.
Please consider spaying your girl after this. This is not the way to begin breeding dobermans. Please check out the threads here on reputable breeding and what all goes into it. https://www.dobermantalk.com/breeding...formation.html
Forgive me if I sound harsh - there are far too many unwanted dobes out there, many of them product of breedings like yours.