Some possible candidates: Cosmere, Malzan, Wheel of Time, Discworld, Star Wars, Star Trek.

Would Moorcock's Eternal Champion count? Multiple different series, but all in the same interconnected multiverse.

Perry Rhodan is a good candidate (a weekly 64 page issue for the last 60 years, plus hundreds of paperbacks and a few spin-off series).

Worm/Ward perhaps?

Would you consider novelisations of a movie series or tv series? Or are you specifically looking for written, non-adaptive works?

Could Warhammer 40k be a good contender too?

Warhammer 40k! With c.1000 published works it beats Forgotten Realms by a lot.

I did a quick word-count of my Star Wars library and it's about 70 million words. That's not including quite a lot of the 'young reader' stuff that's out there. I'd guess another couple of million words at least

My Star Trek library clocks in at about 8 million.

@DanielRoseman - Yes, I think that would be like Cosmere.

@fez - Everything goes if you can make a case for it. It's not like this statistic has any actual relevence.

@Valorum ISFDb lists under 1000 works in the "Star Wars Universe" as compared to more than twice that in the "Warhammer Universe" but I have no way of comparing by wordcount.

asssuming professionally published vs fan fictions?

Dragonlance has a ton of books, I'd imagine it's somewhere up there...

@NKCampbell - A fanfic wouldn't count as part of the official universe it's set in, but a stand-alone online self published universe would be okay.

Realm of the Elderlings? 16 novels (12 of them long), plus various short stories?

VTC: The lack of guidance about language in this question makes it unanswerable (e.g., German smooshes what would be English phrases together into single words, languages with radically different orthographies—e.g., Chinese vs English, etc.) make this question an exercise in comparing apples to starfish to transit systems.

For English, the Ring of Fire universe (starting with Eric Flint's 1632) is getting up there.

Battlefield Earth? That was pretty damn long. Or did it just seem that long?

Does this include comic books/graphic novels, or just prose novels? Marvel/DC comics obviously aren't as wordy as novels, but might make up for it in sheer volume.