Hi Sadun,
The number field is part of the data structure bound to the table, I assume. In this case, I would do this in a data-driven way. In the Data changed script of the activity in the diagram, react to the list data changes and fix up all "number" fields in the list by iterating through the list. New items would get their null value populating and incorrect numbers would be updated.
Hope this helps,
Grant.
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Grant Taylor
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