I had 2 one at 19 weeks (policy of the imaging place I went to) and one again at 24 weeks I think because they needed to check the heart because they didn't get a good view in the first u/s. I think the only reason some have more than others is if they have medical issues or there are possible issues with bubs. I believe if you are part of some kind of study they can give you u/s too. You will be told if you need them though, however you can have them if you feel it's necessary but you'd need to discuss that with your dr.

ETA: I haven't heard much about u/s effecting babies, some have u/s all the way through their pg's and they've had perfectly healthy babies.

HTH