I know someone that started out with 3 on her first ultrasound. 2 were smaller and the couldn't find heart beats. When she went back a couple of weeks later, both of them had been absorbed, and there was 1 healthy baby growing.
This happened to me. I had my first ultrasound at 10 weeks and 4 days. There were 2 sacs. One empty and one with a healthy baby and heart beat. By my next ultrasound the second say was gone. I was quite devastated by thankful for my healthy baby girl. 24 weeks and going strong