Just a little back-of-the-envelope estimating... Pocket has been publishing Trek fiction for over 31 years now, and if they'd published only one book per month, that would be 372 books. But from about 1995 (when Voyager
premiered) to maybe 2007, they published two books per month, which adds another 144 or so, bringing it to at least 516. And that's not counting the dozens of hardcovers and trade paperbacks, which push it well over 550. Throw in the 13 original novels and 2 anthologies Bantam did, plus the 13 volumes of TOS episode adaptations and the 10 TAS adaptations, and you're nearing 600.
And that's just prose novels. The DVD-ROM collecting all the Trek comics up through 2002 contains over 500 comics, and since then, TokyoPop has given us four mangas and IDW has come out with about 30 distinct miniseries mostly ranging from 4-6 issues each and one ongoing series that's about to release its 16th issue. So the number of individual comics probably surpasses the number of prose books.