I find this topic very interesting.

Here's my experience with series length.

Series 1: 12 books. Complete. I see pretty much the same sell-through between books 2 and 3 as I see between books 2 and 12, so the length didn't hurt it. I planned for it to be 12 books long from the start, and I wrote book 1 to be standaloneable, so I think most people who go on to book 2 do so with the expectation that they'll be reading the whole series.

Series 2: Will be 18 books, and currently has 6. It was originally going to be 12 books, but the fans love it so much that I decided to commit to it being 18 instead. Every three books is a satisfying pausing point, so I could stop it at any point after a given three-book mini-arc if I wanted to. It's my fans' favorite series, though, and I've already bought the cover art for all 18 books, so I'm sure I'll be continuing it. Sell-through between books 1 and 2 is slightly lower now than when I only had 2 or 3 books in the series, but sell-through between books 2 and 3 and books 2 and 6 is exactly the same. Anyone who goes on to book 2 seems to want to read all the rest of the series (yay!).

Series 3: Will be 3 books, and currently has 2. The first book was written as a standalone, but fans wanted more, so I finally wrote the second book in a trilogy that they've been begging for, and I'll be writing the third book this year. I don't know whether making it a longer series would hurt it or help it, but I know that for creative reasons, I won't be writing a book 4 in this series. I will instead write a second trilogy that has the same branding but is officially the start of a brand new trilogy, and those two trilogies will be easy to read in either order.

Series 4: Will be 3 books, and currently has 2. Even though it's a series that a lot of my readers love and say is my best, the sales are weak, so I won't be expanding it past the original arc, which I was tentatively thinking of doing.

Series 5: Will be 5 books, and currently has 1. I can't say anything yet about sell-through, obviously, but the sales of the first book are good and the fans seem to love it and keep asking for a sequel. I suspect five books will be optimal for it, from both a storytelling and marketing perspective.

Series 6: Will be 6 books, and currently has 3. The first three books are a complete arc, and the next three books will be a complete arc with new main characters, so I'm pausing it for the moment in order to work on other series for awhile. It's not one of my better-selling series, which is why it's not a priority to write the second arc right now, but because I really want to, I will write the second arc when I have time for it.

Series 7: Will be 18 books, and currently has 11. I saw a lot more sell-through between books 1 and 2 when I had only four or five books in the series, and the difference in sell-through between books 2 and 3 and between books 2 and 11 is about half, so there's a noticeable dropoff in the middle. Not in any one particular place, but usually somewhere in the vicinity of book 5. Unlike my other series, this one is a serial with cliffhangers, so I think my readers are more willing to continue a long series that ends in satisfying pausing places than a long serial that has cliffhangers. (Which is understandable because I would feel the same way. This story just can't be told in any other structure than the one I'm telling it in.) On the other hand, this world is SUPER fun to write in, so I have no problem with the fact that I've committed to 18 books. (

.) There will eventually also be a spinoff trilogy and a companion series set in the same world with brand new characters, but I might use different branding for the companion series to see if that works better.

Series 8: 3 books. Complete. I can't say anything about sell-through yet because I'm waiting to release it until I have finished book 1 in a second trilogy that is set in the same world. I am thinking I will release all four of those books on the same day.

My takeaway from this is that I probably always want to have at least one series of 10+ books going, but it's probably better not to commit to a series being 10+ books unless I'm sure I'll want to write that many regardless of how well it sells. Most of my planned series after this point are trilogies.

(By the way, hi! This is my first post here, since I just heard about this forum, but a lot of you probably know me from Kboards.)