It depends upon whether one counts all mediums and all works, whether narrative or non-narrative.
I am WAY behind
Peter David (45 novels) and
Michael Jan Friedman (36 novels), and likely will never approach their aggregate output (especially if we add in short fiction and comics). They are the giants who walk among us; I doubt
Trek shall see their like again.
If we're counting everything, then I'm probably in fourth place behind
Dayton Ward (who, in addition to prose fiction, pens non-narrative
Star Trek tie-in books, such as the Travel Guides and Kirk Fu, etc.) and the very prolific writers who pen the comic books (though that's kind of an apples-to-oranges comparison; I can draft a comic-book script in a matter of days; it takes me months to write a novel. I'm not sure how to weight comics vs. novels for fairer comparison.)
If we count only full-length
novels (and treat collaborative works as a full credit, regardless of the number of partners) and exclude short fiction, I think I'm in third place (
Oblivion's Gate will be my 29th
Star Trek novel, and
Moments Asunder will be Dayton's 23rd).
If we include novellas, Dayton adds 14
Star Trek credits to his total, while I add 7, putting him ahead, 37 to 36.
If you add short stories, his lead widens: I add only 3 credits, while he adds 9: score now, 46 to 39.
If I get credit for co-writing two episodes of Star Trek: DS9, it's 46 to 41.
If you count non-narrative books, I gain 1, while he adds 3: 49 to 42.
Add in comic-books, he adds 2 issues, while I add 4: 51 to 46.
So, unless we count ONLY full-length novels, I am in fourth place behind Dayton. I can say I am the third-most-prolific author of
Star Trek novels, but that's a mighty specific niche....