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

He shows a couple techniques.

Roy Tamir wrote:

I am using excel for a warranty claim form I created. Need a unique numbering
system to create a unique identification number per form as soon as I open
it. Once saved as a filled up warranty claim, I want that field/cell to keep
this number but when I open this form as a template, I want a unique number
created and given to it on the designated cell called 'Warranty claim number
____'

--

Dave Peterson