I am just finishing my first book in a series that I started, and I'm wondering if you need to plan all the books in your series. For reference, I'm not quite sure if my series is going to have two or three books, but I'm hoping it pans out to a trilogy. (This is the first novel I have written, so that's why I'm asking, just FYI).
It depends! Dusting hands off with a smile of satisfaction.
Ah, you ask, "Upon what?"
Mostly upon what you as the writer want to do. Where you want to take your characters. How you want to explore the theme(s) in your writing. How you want the reader to feel at the end as well as along the way.
It is quite possible to write a whole series of books set in the same world in which the main characters hardly evolve. The books are loosely coupled, perhaps to the point that the reader can consume them in any order. In such a series, minimal planning is required.
It is also possible to write a series in which major events in the later books are foreshadowed throughout the earlier books. Characters may change dramatically through the series and events may build upon one another such that there is only one sequence of reading the books that makes any sense. This requires much in the way of planning.
Note that you do not have to know the exact structure of the follow on books, or even the number of those books. But you probably would be well served to know where you want the story and characters to end up at the conclusion of the series, plus the major events that lead the reader through the story.