Arc action to increase quantity of an item when added to a cart -- 4 is arbitrary selected here, but you could substitute this with some other logic or configured variable.

embedded.commerce.carts.addItem.after.js

var CartItemFactory = require('mozu-node-sdk/clients/commerce/carts/cartItem');
var CartFactory = require('mozu-node-sdk/clients/commerce/cart');

function itemUpdated(item) {
    console.info(item);
    callback();
}

module.exports = function (context, callback) {
    var cartItemResource = CartItemFactory(context);
    var cartResource = CartFactory(context);

    var cart = context.get.cart();
    var cartItem = context.get.cartItem();

    cartItemResource.updateCartItemQuantity({ cartItemId: cartItem.id, quantity: 4 })
    .then(itemUpdated)
    .catch(function (err) {
        console.error(err);
        callback();
    });
}