It's varied with each pregnancy. With my oldest I started having monthly, and then weekly u/s starting at about 20 weeks. I'd had several before then, as well. I'm not sure how many in total.
With the next one I had a dating u/s because we didn't know when I concieved, and then a few more in the office to check things out. Then I started having monthly growth scans at about 25 weeks, and bi-weekly BPP's until I delivered.
With #3, I had a couple of early u/s's to check things out. a 20 week growth scan, 2 scans to check the pressure in the cerebral artery (I'm rh sensitized), and then she never made her kick counts, so I had multiple BPP's a week.
I'm high risk with various issues going on, so that's why I have so many. I usually have an early one, or a few, just to make sure I'm not going to have another m/c. And after the age of viability I have them a lot so they will know if they need to deliver me right away.
Wife to my Navy doc, SAHM to 4 (11, 8, 5, 1)