There’s enough of them that inbreeding isn’t really an issue, at least in Indiana . It’s almost impossible to join the Amish from outside; even if you managed to learn the language they would always view you as an outsider. I know of only one guy who pulled it off. Why, I haven’t a clue. My dad’s family left when he was a teen, thank God!
There are a few Mennonite offshoot sects where I grew up that their numbers have dwindled and they refuse to marry outside of their group. Their kids are definitely looking a bit cross eyed these days.