It depends on your doctor and your previous pregnancies. With my first son I had probably 2-3 u/s. One to determine dates, one mid-way (20 weeker), and perhaps another one.
My second pregnancy I had 3 u/s but that was because of problems that ended in miscarriage and a D&C at 11 weeks.
My third and current pregnancy I have had SEVERAL u/s due to placenta previa and other concerns.
My Dr is also great about giving the mom piece of mind. If I feel the need for an u/s he is very understanding. If he feels it isn't warranted he will ask if a doppler HB check is sufficient. If I don't feel like it is he is fine with proceeding with an u/s.
It isn't abnormal to only have one u/s in a pregnancy but you should be able to request more if you feel like you want/need them. You can also go to an u/s place (ie: Fetal Foto, etc) but they can be pricey and you need to know that the people doing the u/s are NOT REGISTERED U/S TECHS.
Blessings on your Pregnancy!!
A.