how to find the serial number for iWork ´08

The Apple registration servers that could validate any serial number for installation from an iWork 'YY DVD have been switched off for at least seven years now. You are done with that DVD. Pages versions introduced since Oct 2013 (v5.0 — current) can open most Pages '06, '08, '09, and all subsequent Pages document formats.

If you are referring to an AppleWorks DVD, it cannot be installed on anything newer than Snow Leopard (10.6.8), and again, no servers to validate any DVD serial number.

As far as I know no iWork software has ever had individual serial numbers. Why do you think you need one?

Why do you need a serial number for a product long since discontinued, and no longer sold by Apple? Any such serial number, if it ever existed, would be on the original DVD, and not the inherent applications.