If you're willing to accept individual issues of comic books as "one book" then DC has published over 40,000 issues, making it the winner by a huge margin. Though, not all of those issues take place in the same universe -- DC (and Marvel) comics take place in a multiverse where not all stories are directly related. Even if you just narrow down to the "mainstream" story, one of these two would win by a lot.
Otherwise, if you're willing to accept novellas or pulp magazine formats, the Perry Rhodan series is in the thousands, at least, and still going.
If you are specifically asking for full length novels, then I think the winner might be The Forgotten Realms, which I count over 200 novels (not counting the numerous short story anthologies), and again, more to come.
If you mean works by a single author, the biggest one I'm aware of is The Riftware Cycle by Raymond E. Feist, which is up to 30 books and (I believe) still going.
If you mean the largest series telling one single, continuous unbroken story, I'm less clear, but I know that The Wheel of Time got up to 14 novels, plus one novella; I've never personally read anything longer.