|I don't want to use the same document number twice and every time I open this spreadsheet, the document number is updated automatically as the NOW function does.|
=NOW() will recalculate whenever any formula within the entire Workbook recalculates
When you first open the spread sheet it will be 1
When you enter any data in any cell it will become 2
When the sheet recalculates it will become 3
I don't think that is what you want. Is It?
I believe what your looking for is a Unique ID number that will be inserted, the First time the record is created and thereafter remain static.
If that is what you need then your best bet is a Macro.