откройте catalog/view/javascript/facebook/facebook_pixel.js найдите

cart.add = function(productID, quantity) {
 fireProductInfoEvent('AddToCart', productID, quantity);
 oldCartAdd.apply(oldCartAdd, [productID, quantity]);
};

замените на

 cart.add = function(productID, elem) {
 let quantity = $(elem).find('.qty-switch__input').val();
 
 if(typeof(quantity) == 'undefined') quantity = 1;

 fireProductInfoEvent('AddToCart', productID, quantity);
 oldCartAdd.apply(oldCartAdd, [productID, elem]);
};