This page can be accessed by going to System > Settings > Checkout
invoices numbers generated manually by seller (administrator) on order page. Do not confuse Order ID with Invoice ID (number)
Invoice Start No
Set the starting number the invoices will begin from. The next invoice you create will have the new invoice number.
Invoice Prefix
Set the invoice prefix e.g. IN#. Invoice ID's will start at IN#1 for each unique prefix.