I'm confused about ultrasounds..

I see so many people have at least 2-3 ultrasounds around 17 weeks or under. I've only had one... and that was to see if there was a baby. Im considered high risk. (I have a issue that could result in a c section, losing blood supply to the baby, or early delivery) I don't get another ultrasound till a month from now... I'll be 20 weeks and one day by then. I don't understand... I'ts not that I want to have more ultrasounds, it's that I'm high risk and would like to know how everything is going. If my tumor is growing. If the placenta has enough room. Etc. should I be concerned?? I have good insurance.