Hi,
In France and probably in all Europe is mandatory to have consecutive invoice numbers without any blank.
It's very important for NOP commerce developpement to be able to get rid of "false" orders ( incomplete, unpaid orders) that generate an invoice number.
The invoice in French laws (and probably European laws), must be generated when the order is sent, not before ,not after.
It's very important that NOP developpers find a solution to cope with that problem.
You can read that thread for better understanding:
https://www.nopcommerce.com/boards/t/48608/invoice-when-order-status-is-sent.aspx#192794