Wow, this is the first time I hear the word "doulogy"... lol, sorry for my ignorance..
So far, I didn't hear any story in 2, 4, 5, 6 books. It's always a trilogy for long novels. Is there a reason behind that I might have missed?
There are plenty of series in 2, 4, 5, 6, 7 or even 20 books. It's true that trilogies are pretty common and always have been - I think there's an aesthetic reason for that (the shape of a story in three parts is a fairly tried and tested one) and also all kinds of commercial ones (successful precedents like LOTR or Star Wars; it's nice for publishers etc to be able to sell three books or movies instead of two, hence the odd decision to stretch The Hobbit out into three movies, for heaven's sake.)
One of the most popular "trilogies" of fantasy books, The Lord of the Rings by J. R. R. Tolkien, is not a trilogy, though it is often referred to as such. Tolkien regarded it as a single work and divided it into a prologue, six books, and five appendices. Because of post-World War II paper shortages, it was originally published in three volumes. It is still most commonly sold as three volumes, but has also been published in one-volume and seven-volume editions (six books and the appendices).
Wikipedia article does address LOTR somewhat:
Next question:
When you write a book, do you already know how many books it would take? Do you always plan for it's length or, it depends on the flow of the story?
For my WIP, I initially planned to make a trilogy but could also be 2 or 4 books depending on the the plot. New scenes and ideas always comes to mind thus making the story more longer but interesting...
It's the magic number.
So far, I didn't hear any story in 2, 4, 5, 6 books. It's always a trilogy for long novels. Is there a reason behind that I might have missed?
I see more Trilogies in SFF.
But, as long as the story tells for, that is how long the novels should be.
Trilogies are kinda attractive because you can have a opening, a middle and a end.
If we're talking about numbers, what about a 120 book series? All written by the same author too!
Lets see if any of you can guess it.