average_amount_by_survivor:{$divide: ['$itens.count', countTotal]}

.get(function(req, res){
    Survivor.count(function(error,countTotal){
        Survivor.aggregate([
            {$unwind: '$itens'},
            {$group:{
                //itens:{$push: '$itens'},
                _id: '$itens.item',
                total_amount:{$sum:'$itens.count'},
                average_amount_by_survivor:{$divide: ['$itens.count', countTotal]}
            }}],
            function(error, result){
                console.log(result);
                console.log(countTotal);
                res.json(result)
            });
    });
});