You can create one invoice from more than one delivery note, click on the Create invoice option from the bulk options menu. 
Select delivery notes  from the list, and then click on the [Create invoice] button. 
In order to create invoice from more than one delivery note, the delivery notes should have same values for the following fields:
Select the invoice currency and then click on the [CREATE] button.
Items for all delivery notes are listed together in the invoice.
- When you approve a quote, a stock reservation is made for the items quantities, and it will not affect the stock.
- When you approve an invoice, the products are automatically taken from the stock.
- When you approve a delivery notes, the products are automatically taken from stock, and when create an invoice for this delivery note and then approve it, it will not affect the stock.