You could do something like this:

function getQuestionNumber(){
    var questionComponents = Adapt.assessment.get()[0]._currentQuestionComponents;
    return questionComponents.indexOf(Adapt.findById(this._id)) + 1;
}
Handlebars.registerHelper("questionNumber", getQuestionNumber);

That would allow you to display the question number by setting the displayTitle of the question to "Question {{questionNumber}}" in the JSON