One thing I can't stand is when authors string out a series for no reason other to sell books. For example: The Wheel Of Time by Robert Jordon. This was one of the first fantasy series I really got into. It had an extremely good start, and would have made a fantastic trilogy. Instead, Jordon turned it into a 12 volume behemoth, mired down in details. Then, when he finished book 11, he DIED! Personally, I made it to book 6, then quit reading it all together.
I'm thankful that J. K. Rollings didn't do this with Harry Potter. Although a few of her books, especially book 5, could have been a hell of a lot shorter.
My series, I am going to limit it to 7 books. Five in the main story arc, and two collections of short stories.
Blood Of The Righteous (Published and available in both physical form and e-book)
Fall Of The Righteous (Writing this one now)
Redemption Of The Righteous
The Ki Kalendeen Chronicles - A Collection Of Short Stories
Return Of The Righteous
The Ki Kalendeen Chronicles - Tales Of The Blackguard
Wrath Of The Righteous
Afterwards, I have some ideas for a trilogy involving one of the minor characters from the first series.