Number of Ultrasounds

I was just curious how many ultrasounds is normal? My doctor did one at 6.5 weeks called a dating ultrasound and then I don't have another one until about 17-20 weeks for the anatomy one. Just wondered if that is the norm and if maybe I should request one inbetween those two? We wanted to announce at 15 weeks on our Christmas card but it worries me doing that and not having another ultrasound before. Thanks!