One approach to solve this would be to display a popup where the user can specify an "Amount". Here is a complete example in the modeler. You will need to create another entity to hold the amount attiribute. In my example this Entity is called "ProductOrder".
Here is the microflow you see in the screenshot so that you can see what happens insude the two actions. Basically you need to make sure to set the assiciation between product and productOrder and then pass the newProductOrder object to the popup page. Also make sure to use the popup layout .
There are probably other better solutions out there but this is pretty simple and it doesn't use datasource microflows.
I hope this helps,