Dennis @ PerfectForms
An instance number cannot exist until the instance has been created. An instance is not created until the form is submitted the first time. I would suggest creating a new first stage and make the first submit a “submit and re-open same”. Then when the form opens use a Simple Branch behavior to determine if the instance is in the second stage. If True then set you order number field and submit. If False go to and Exit behavior.
There are many ways to handle this problem, but that is what I would do. It is probably the most simple solution.