To be able to create invoice numbers in a sequence that follows Sales Order numbers you would need to arrange the Sales Order numbers first before you hit 'Submit' button to bill these orders.
Below are the steps when billing Sales Orders:
1. Navigate to Transactions > Sales > Bill Sales Orders.
2. Click Mark All button or manually select the orders you wish to bill by checking the 'Invoice' checkbox.
3. Click 'Order #' column header to arrange it in ascending order.
4. Click Submit.
5. Example Result:
o SO05 > IN10
o SO06 > IN11
o SO07 > IN12
o SO08 > IN13
NOTE: This does not work when billing more than 10 Sales Orders.