I heard that soon it will no longer be possible to add additional pages to a passport, so what do I do if I run out of blank pages before my passport expires?

Yes, you heard correctly. Starting in 2016 the passport agency will discontinue the practice of adding pages to an existing passport as this is thought to represent a vulnerability. If you run out of pages before your passport expires you will need to apply for a new passport. New passports come in a 28-page and a 52-page version. If you reside overseas and have your passport issued by an overseas embassy you will be issued a 52-page passport automatically. If you reside in the U.S. and have your passport issued here the default is the 28-page booklet unless the 52-page version is requested. If you travel internationally frequently be sure to request the 52-page booklet on your application form.