If you're looking to auto-increment invoice numbering, take a look here:
http://www.mcgimpsey.com/excel/udfs/sequentialnums.html
As for linking from one cell in (any) Excel workbook to another cell in
either the same workbook or another one, that's easy to do:
If multiple workbooks are involved, open both/all of them.
Start in the workbook that's to have a copy of the number, choose the cell
and type the = symbol.
Then go to the other workbook/sheet/cell with the number to be 'copied' and
select that cell and simply hit the [Enter] key. The link is created.
But each invoice would need to be linked to a different source cell, because
if all invoices link to the same invoice number cell somewhere, then when you
change the source invoice number, all invoices linked to that cell would show
the same number.