Upvote:2
It was (indirectly) stipulated in 7 FAM 1300 Appendix V, Supplemental Visa Page Inserts:
a. Each set of supplemental visa page inserts (also known as "visa page inserts") adds an additional 24 pages to a passport. They extend the useful life of a passport as an accommodation to individuals who travel frequently to countries requiring visas.
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c. The Bureau of Consular Affairs (CA) recommends adding no more than two sets of visa page inserts to a passport for the reason that the added thickness may result in damage to the binding of the book during its validity.
Since US passports can have at most 52 pages without extensions and you can get at most two 24-page "visa page inserts", the total maximum number of pages should be 52+24*2 = 100.