Lets say a landing pages are:

http://www.xyzcompany.com/?123
http://www.xyzcompany.com/?456

and thank you page for registration is
http://www.xyzcompany.com/thankyou.htm

To track campaigns landing on the different landing pages and to measure ratio of registration, would we need to set up separate goal funnels or simply set up the thankyou.htm as a goal and would be able to view the ratio of the two landing pages there?

Why don't you just keep it simple and create a different thankyou page for each and set a different goal for each thankyou page?

If it is the volume of test pages you are testing, remember, Analytics only allows 5 goals. You may be better off MV testing with website optimizer.