I'm not sure about the numbers of Christians, but I think there were a couple of hundred. I followed up on that back in 2015 and 2016 through the state department, and we were able to get them some support. ISIS refused to negotiate about the Yazidi women and children, because they considered Yazidis infidels worthy of enslavement, but they negotiated about the Christians, and we were able to get them released.
On November 20th, 2018. See this statement in context.