You may want to look at how J.E. McGimpsey does it:
http://mcgimpsey.com/excel/udfs/sequentialnums.html

He shows a couple techniques.

Decatur Aggie wrote:

I am developing a form in Excel that will have multiple users across several
facilities. Each time a user accesses the form, the form needs to have a
unique incremental number, so the form can be referenced for use later. I
was thinking that a counter would be the easiet function, but have been
unable to find anything or create anything that would create this number.
The number must increment by 1 each time the form is used - ex: 10001, 10002,
10003, etc... Does anyone have any ideas on this?

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Dave Peterson