I think right now it is a several hundred thousand, like people mentioned.
A movie would bring back a lot of former fans. I noticed some months ago, Cartoon Network started airing reruns of Goosebumps. So many people my age (19-20) watched the reruns because it took them back to their childhood and everyone was so excited to see it on TV again. I think something similar would happen with Animorphs if a movie was made.