Adding Line items numbers to a record....read on.

I have an existing list of line items in a table. Thru a pipeline these lines items are bulk added to a Parent table Order header record. Is there a way to assign a unique line item #'s to each order's line item, starting with line item # 1. (so can't just show record id for each line item). The catch here is that that the user may add a line item individually (not in batch with the pipeline) to the order and that added item must equal the last line item # plus 1.