I used to live close to the Old Order Amish in Pennsylvania, and as far as I ever heard, none of them left the order. The Amish boys would be allowed as teenagers to run free in the evenings after the chores were done, and there was a custom called "throwing your hat" where a young man would leave the farm through the back field, but hang his hat on the back fence so his mother would know he wasn't lost or in trouble. On the way back home he could collect his hat. I got this story from an Amish boy, so I know it was true.
I going to raise a barn or two with the community, take the buggy to Ezekiel Lapp's Amish Buggy Works for some new springs... and get with my neighbor, Jedediah Eaby and discuss how early we will till this season !
There is no official number, the Amish themselves don't keep statistics on this type of thing, but scholars who study Amish and other Anabaptist cultures estimate that 15-20% of people born Amish will leave the faith.