I think this should settle the question of how many sets of pages can be added.
Although now with the new ripoff fee structure I do not believe it ever makes sense to add pages anymore. You might as well renew even if you passport is valid for another 8 years because adding pages at $82 for 24 pages costs you $3.41 per page whereas renewing your passport and requesting a 48 page passport yields 48 pages for $110 which is only $2.29 per page.