Just Curious.

For the authors who have written a series, did it start out that way? Was it planned from the beginning that there were going to be three or more books, or did it just develop? 
I am writing the Aftershock series, planned from the beginning to be at least three books. I didn't see how I could include everything in one book. When I pitched the first book to an agent at a writer's conference, she thought it could develop into a five book series. Well, what do you do when its difficult to find readers for that first book? Do you continue with the series? Or change course?
I love the story myself, but is that enough? I don't write to follow trends, I write for myself, what interests me at the time. You may see from my book page that I have written action/adventure/thriller types as well as a biographical western and now post-apocalyptic. 
Give me your thoughts.
Thanks,
​Mike