How many alter sounds??

Kari • Married to my best friend of 7 years and high school sweet heart. So excited that we have our first baby on the way!😍
How many alter sounds do women usually get?? I've only had two. One at 9 weeks and 4 days. And the other to find out the gender at 20 weeks 5 days. 
I would like to see my little man again before he is born. I kind of wanted to try a 3D one. 
Do doctors usually give you another alter sound appointment? Or do you have to request one?