J.E. McGimpsey has some suggestions:
http://mcgimpsey.com/excel/udfs/sequentialnums.html

Lloyd wrote:

I have created a Ticket Form for billing purposes. They need to generate
sequentially, but after the ticket is filled out and saved, the number cant
change when its reopened. Would I have to use another program to do this or
could I get excel to work this out for me?

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Dave Peterson