I want to ask how many times an anime/sci fi live action which is in pre-production have changed directors already. I've read other topics here about making production questions (like behind the scenes) but I don't have it clear. Is it allowed?
Behind-the-scenes information is explicitly on-topic according to the help centre, and so is "trivia" such as how many times a film has changed directors. This kind of information is part of what a fan might be interested in, and it's on-topic to ask about it (provided it's sci-fi or fantasy, of course).
While the subject is on topic (see Rand al'Thor's answer), simply asking for a list of movies that have changed directors during pre-production may be closed as off topic because it asks for a (potentially very large) list of works. Try to phrase the question so that it can have a definitive answer rather than asking for an open-ended list.