After December 31, 2015, there will be no more extra visa pages added to U.S. passports.
Instead, 52-page passport books will be issued to U.S. citizens applying from outside the United States.
Renew your passport early to make sure you have enough blank pages for visa stamps from other countries. Some require two blank pages for entry and exit.
See travel.state.gov for more information on entry, exit, and foreign visa requirements.