Part of it depends on your insurance. Most will only cover 2 ultrasounds unless it is medically required for more. With my first, I had a few more because she was breech. They kept checking to see if she turned, but never did. There are places out there to have them done on your own,like for the 3-d ones.
Depends on your Dr. Many places do the last one at about 20 weeks (the anatomy scan) unless there's a problem or concern, then there are more