I bought Pages but didn't get serial no. to key in, how can i do?

You don't need to key in serial number, if you purchased retail box.

If you got Pages online, serial number is sent via Email from Apple Store.

Getting the request for a serial number means that at some time you had the iWork ’09 trial installed. You need to delete the trial & then reinstall from the boxed DVD or the Mac App Store. The files to delete are the iWork ’09 folder from the main HD > Applications; the iWork ’09 folder in HD > Library > Application Support & the individual iWork application plist files found in HD > Users > (your account) > Library > Preferences for each user. The easiest way to fix the problem is to use Yvan Koenig's AppleScript that removes the files. Yvan's box.com account often is out of bandwith. I have put a copy on my Box.com account in the iWork folder. Download uninstall iWork '09.zip, decompress it, open it in AppleScript Editor and click the Run button.

Apple's support article on the subject says it's due to having a copy of the trial that is a higher version that the one you've purchased, but I'm not sure that is very common. This problem started with Snow Leopard & it seems to be some code in the iWork installers that doesn't do what it should.