A word count is also a signal to the student of what level of depth the teacher is hoping for. For most interesting questions you could write anything between

A word count is also a signal to the student of what level of depth the teacher is hoping for. For most interesting questions you could write anything between a few sentences and a full book, depending on how much you develop and defend your arguments, evaluate other possible answers, etc. The word count is a signal on where on that spectrum you should be aiming.