Dennis @ PerfectForms
Instance number is the right metric for what you have described. The trick is to use a Submit and re-open same instance at some point before the user submits the instance. You can even do it on when form opens. Here is how:
Drag in a hidden helper field. When the form opens use a Simple Branch object (is the hidden helper empty?). If false go to an Exit behavior object because the form instance has already been opened. If true use a Set Field object to put a “1” in your hidden helper and then use a Submit behavior object (check the radio button that says Re-Open Same Instance).
Finally make sure that you account for this in your workflow. If you are using “always if no other condition is met” on the join paths between the first and second stages you will have to add an additional stage to account for the first submit. Otherwise simply create a condition to move from the first stage.