How many US did you get?

I am 10wks 5days with my second baby and I just got my second ultrasound yesterday. The first was when I was 6wks 4days to check how far along I was, and this one was to check for genetic testing! My next one will be when I'm around 18wks so they can check the gender! Some people say they only get ONE u/s when they are 20 wks?? That sounds crazy because how will they know if the baby is healthy before that?