Could someone help, please?

I'd like to have an auto number field based on certain criteria.

I want to count a delivery note document. The delivery may happen in multiple actions. 

If I edit the record in the same action several times it should not influence counting.

For example:

In delivery1 action I open the report for editing and update two qty fields. It means +1 in counting. Later I open the record for another editing, but this is still delivery1 action, so there is no counting etc.

Next time I open the record for editing delivery2, then it means +1 in counting again.

The relevant fields are ordinary form fields and not subform fields: 

Field names: delivery action, serial number

Do I need number field, or single li field is OK?

Of course I do not want to count only one FormA, but FormB, FormC etc.

Example:

Delivery action                              serial number

FormB action1                                          1

FormB action1                                          1

FormA action1                                          2

FormB action2                                         3

FormC action1                                          4                  etc

Thanks, if anyone can help!

Best wishes