At what points in your pregnancy do you get all your ultrasounds?

I asked a Q on here a few days ago about being 100% of the because I had my anatomy scan u/s at 18 weeks the other day and the tech *thinks* it's a boy but sounded vague and I want to know for sure, so some people said to wait a couple weeks to get the other u/s, but I asked my OB yesterday and he said I don't get ANY more u/s's. At Kaiser (my HMO) they do one in the 1st trimester and one at 18 weeks and that's IT. He told me no more at all throughout the pregnancy unless there was something medically wrong. So now I'm wondering what do most doctors do? Are you all getting one at 20 something weeks also??? How many u/s's are you getting altogether???