As of January 1st, 2016, the Department of State is no longer offering additional pages for U.S. passports. Previously, U.S. passport holders had the option to pay for the insertion of an additional 24-page visa insert. The decision to discontinue this service was made to enhance the security of the passport and to abide by international passport standards. Americans who need additional pages in their passport to travel should apply for a new 52-page passport (most countries require two empty pages for a new visa, and there must be room in the passport for entry/exit stamps for arrivals and departures while traveling).
In Guangzhou, it normally takes about two weeks to process and receive a new 52-page passport, so travelers should plan in advance to apply for a new passport when they have only a few blank visa pages left.