Forms: Invoices - Include line number

I wish to use a customized form to print Invoice. I want to add Line Item Numbers (Sequential numbers , 1, 2, 3... ) to identify each line on the item list as there might have 20 items per inovice.

Is there a way to add this field to the form layout and populate it with auto generated line item numbers? 

I think it would be good if you could add the line number of the transaction to the Invoice table.  Our invoicing can have over 20 transactions and we would find it easier to reference a line with a number.