Hi,
First, note that the format of the invoice number can be tweaked with the "Invoice number format" setting of the Hikashop configuration page:
www.hikashop.com/support/documentation/5...nfig.html#main_order
Supposing that you use {id} or {order_invoice_id} in the format, it means that the system will use the "order_invoice_id" value as base for the invoice number.
For example, if you have the format FA-{id}, and the order_invoice_id is 908, then, the invoice number will be FA-908
And regarding the order_invoice_id, to generate the next one, the system takes the biggest one among all the orders already in the database, and increment the value.
So, if you want the next order_invoice_id to be 908, you should make sure that no existing order has an order_invoice_id greater than 907 and that at least one order has an order_invoice_id set as 907.