I'm currently on the second draft of my MG fantasy novel, which will end up as a series (if sales do well on the first book, of course." My original plan was to do a four-book main series and then a two-book prequel, but now I feel as if I could add one or two more books to the main series, because a) I have more plans than I think will fit it only four books, and b) I feel like readers would get more attatched to my characters and world if I had more books and events leading up to the end. When I begin querying literary agents, should I mention that I plan for this book to be a part of a series? If so, should I say "4-6 books in the main series" or "undetermined?"
No, you should not mention it.
Near the end, you can mention you believe there is the potential to create a book series with your character, but I'd leave it that.
Just focus on selling the one book, one story. That is all the agent will try to sell, and if it is worthy of a series, they will realize that on their own.
Because if you don't sell the first book, you don't have anything.
Agents and Publishers don't buy series that are in your imagination, they buy completed, polished, edited finished books in their final form that require minimal effort on their part to put into production.
Rex Stout published 33 Nero Wolfe books in his series, one book at a time.
Sell the story you got. You won't get any credit or extra consideration for talking about how many more books you can write. It's like talking about what great marriage material you are when asking somebody out on a first date.
If you even get to the point of an agent or publisher reading your book, they are professionals that will see the potential for a series on their own.
I strongly recommend you don't mention a series at all. You have a very limited number of words in the query letter, the format is quite strict. Spend every word trying to sell this one story. Not trying to plan our your future together.
When querying agents or publishers, you should display your skills and competence as a writer.
It is part of a writer's ability to know what they are writing: a standalone novel, a multi-volume work, or an open-ended series.
Decide what it is you are writing and market it as that.
What you are writing should follow from the story.
Some stories are long, some are short; some have one arc, some have many; some require a single viewpoint, some require several; and so on.
Decide which form fits your story best and write that.
Another skill of a professional writer is flexibility.
If your market demands sequels, you need to be able to expand your standalone novel into a series. If your market demands a love story, you need to be able to incorporate a love story.
Know your market. And be open for the publisher asking you to write something else.