At the end of 2015, the U.S. stopped adding extra pages to full passports. If your book runneth over, you can renew and choose a 52-page document instead of the standard 28-pager, for the same fee. Or, shoot for a duplicate.
It’s a little-known fact that the U.S. Department of State will issue two passports to very frequent travelers—like airline pilots or journalists—who may have to apply for multiple visas at one time. The process is the same as for a renewal (including the fee); however, you also have to submit a written, signed statement explaining why you need an extra passport and promising that, if you lose one, you’ll report it as soon as possible.