How is this data entered? Is this a manual process or some automated task?
You say internal use only, how is the data used?
I would suggest creating a separate field that displays a 1-n relation data you enter in either a custom table or custom class. This way you can also enrich it later with data like 'expected delivery date', 'status', etc...
You could create an automated task that would update the actual stock when ever a new batch is entered, the values will be combined so you don't get out of sync data.