Usually two. One to see how far you are and one for anatomy. I paid for a gender one then had 3 from hospitals and doctors office so far but really only one is necessary unless there maybe a defect. Then you will neef a level II. I have had to have a level II and trust me. I rather only have one ultrasound than to sit a worry about the baby. Everything was fine but the 2 weeks of a wait was scary.
Ultrasounds are actually terrible for determining how big baby is. They can be off by a pound.