NAIA saw 27 million passengers in 2010.
In 2011, NAIA handled 29 million passengers. That's way up there as one of the busiest in Asia.
My best guess would be that you will arrive into terminal 1, but check your arrive details to be 100% sure.
Here's a map of the Terminal 1
This site also has good airport information in addition to loads of other helpful stuff. Well worth taking a look at the links and the useful tips of almost all aspects of visiting Philippines.
To discover about NAIA Manila follow the link to 'Transportation', then 'Air', then 'Manila Airport Guides'.
Enjoy!!