Why only 2 u/s?

I went to the dr. yesterday bc I had some concerns. I literally begged her to do an u/s on me & she agreed although my first u/s was scheduled for 4/21. The u/s tech told me she was going to cancel my u/s on 4/21 which kind of made me sad! I asked when I would come back & she said around 20 weeks!!!! Wth why do we only get 2! How else do they make sure the baby is okay? I don't like this :( can I request more?