how often to miscarriages happen?

I was just wondering, because I see a lot of woman on here that experience them, which is horrible! I am so so sorry to anyone who ever has to go through that, my eldest sister just had one not too long ago and I remember her calling my mom crying and asking why it had to be her baby. I even cried bc I felt so so bad for her, and her lost baby! I'm only 15, but I just wanted to know how often they happen? Is there anything you can do other than a healthy lifestyle to prevent them?