The official press release announcing the policy change is available here:
In light of this change, SOS reminds all touring personnel of the importance of keeping track of available visa pages in order to avoid delays in visa processing or problems when entering a foreign country. While specific requirements obviously vary depending on your tour's routing, a general suggestion is that active touring U.S. citizens should maintain at least 6 completely blank visa pages (no stamps) in your passport. When you start running low (below 10 blank pages) please make a plan and identify an appropriate time for a renewal.
As a reminder, the last few pages in a U.S. passport are reserved for endorsements or amendments and do not count as visa pages.
Additionally, SOS reminds U.S. citizens of the option to obtain a 2nd, concurrently valid U.S. passport, which offers added flexibility for obtaining visas while touring / traveling internationally. The U.S. government will issue travelers with a 2nd passport, valid for 2 years, on a discretionary basis. For more information, please refer to our website.