That's my unofficial count, obviously. (Palladium Books places the count closer to 95.) 88 books! That's not including extraneous stuff like novels, coloring books and art portfolios, and also not including the 65 issues of The Rifter published to date, practically all of which incorporate official and semi-official material for the game. It's worth noting that some of those 88 books, like the Book of Magic, the Vampire Sourcebook or the Game Master Guide -- which might more accurately be called an arms and equipment guide -- collect or reorganize material that was previously published. (That number also doesn't include the out-of-print oddity known as Rifts Manhunter, the only book made for the game that wasn't published by Palladium Books.)
Especially for those of us that tuned out some time in the late 1990s, the fact that Palladium has been (more or less) steadily pumping out Rifts material for almost a quarter-century, all for the same edition of the game, is surprising. However you feel about the game or the company that publishes it, 88 books has got to be a record for sheer number of gaming materials published for a single iteration of a roleplaying game.
Before you ask, no, I don't have all 88; I have closer to 10 these days. Still, there is some small part of me that wishes I could "catch 'em all".