Congrats on the postitive confirmation of pregnancy for your girl! How exciting for you.
How far along is she? I have heard that ultrasounds are great for confirming pregnancy, but not totally accurate with a puppy count, especially in a larger litter. An x-ray done a week prior to her due date will give you a better indication of a head count as well as for determining their sizes in relation to the dams birth canal.
Good luck to you!