Tables :: Come Up With Automatic Line Numbers For Each Specific Order?

I am trying to come up with automatic line numbers for each specific order. So for example, I have job number 123456 that has ordered 3 items, what I would like is that item 1 has a field with a 1 in it automatically, and item 2 has a 2 in the field and so on. But the trick is that when order 123457 gets entered and has 5 items entered, I would like it to start over at 1 and go to 5. Does this make sense? Is it possible to do this in a table? Or can this happen on the form? That I assign a value to a field. I am using Access 2010, have been for a few years now, but I have lots to learn.