You guys all seem to be forgetting book 10. Would an Animorphs movie really be an Animorphs movie without at least some mention of the Chee? There were a lot of missions to which Erek was essential.
I agree that there needs to be AT LEAST four movies. Probably more like six. Yeah, six is a good number.
Movie 1: #1 and #4 (maybe some mention of Chapman's sacrifice for his daughter, but the bulk of book #2 isn't really needed). The first movie should contain less material so it can go more in-depth with detail and character development and such.
Movie 2: #5, #6, #7, #8, and #10. Maybe not the entire plot of all those books, but certain points need to be hit; Marco's mother being Visser One, Jake being taken, the Ellimist and the Kandrona, Ax contacting his home world, and of course meeting Erek. This movie could be less detailed and more about action than the first one.
Movie 3: #13, #15, #17, #18, and #19. Once again, or at least elements of all those books.
Movie 4: The David Trilogy could really be its own movie. And it probably should.
Movie 5: Skipping over a few books, here. Maybe include just the essentials, like #23, #29, #30, #33, and #38.
Movie 6: This would basically be the final arc. #45-#54. Maybe a few plot points from the final arc could be thrown into the previous movie, as foreshadowing or building action. And then the last movie has got to be epic. Maybe one of those three-and-a-half-hour movies, to fit in all the action and trauma and character development and everything.