I know there is a way in Excel to have a document be numbered with a
unique sequential number each time it is opened (or better yet, each
time it is opened after having been saved). (For example, let's say
that I want to create a purchase order form that can be used by
multiple people. I would like the PO to be automatically and uniquely
numbered each time it is opened). Can anyone provide the formula to do
so? Thanks!

John