I like these (long) series:
Overlord, 13 books.
I think this series just keeps getting better.
Lazy Dungeon Master.
Funny series, I never get bored with it.
Campfire cooking in another world, 10 books.
This series is consistently funny. Book 10 is really nice, too.
Death March, 14 books.
Nice series, but there are too many childish characters in the books and that's getting worse with each new book.
Konosuba, 14 books.
The last few books aren't that great, I think, but up to book 10 or so it's funny. The spin-off Konosuba: Explosion with 3 books is also funny.
Skeleton knight in another world.
Epic story about a (skeleton) knight fighting horrible nation-scale evil in another world, using large-scale over-the-top magic.
So I'm a spider so what, 12 books.
I like this series, but it's very confusing to read because the timelines are so messed up that it's actually annoying. I like the over-the-top action in this series so that makes it worth it. You can just ignore the messed up timelines. In book 10 or 11 there's a lot of side-stories that are so different from the main story that it's like a completely different book. I don't like it. The last book 12 also has a lot of different stories but this time it's done well, because they're back to point for the main story and I enjoyed those. The only thing I don't like is, that the series gives many hints about the ending. I think that's just frustrating as a reader, because every time I think it's the final book and then it isn't ! The end is not near.
Reincarnated as a slime, 11 books.
Nice series about a city/nation-building reincarnation with over-the-top powers. It gets quite absurd in the last few books, though... what with a dragon-powered-dungeon and all... I'm not sure where this series is heading to be honest.