Varies. Most women will have one ultrasound in the first trimester to verify dating and possibly as part of a special early screening for chromosomal abnormalities. A detailed second trimester ultrasound (around 20 weeks)is typical to look for any fetal abnormalities. Any additional ultrasounds will vary depending upon the specific circumstances of your pregnancy.
At least 1. Assuming an uncomplicated pregnancy you should have at least one at 18-20 weeks to confirm normal fetal anatomy. Most women also have an early scan to confirm pregnancy in OB office. So two would not be unusual. More ultrasounds are sometimes needed to follow up different conditions or inconclusive earlier ultrasounds.