When analysing a document, how much historical context would be enough, if I only have 500 words?
You need to set the context to make sense of you answer but don’t want to overlard it with too much detail. Be succinct and get the background sorted out iin a couple of paragraphs. You are writing for an informed audience so you don’t have to go back to when the earth cooled. More than this and the examiner will wonder if you have understood the question. Usually the number of words is a guide. If you have more that is relevant to say on the subject don’t be constrained too much, although if you end up doing acres more you are probably waffling. Perish the thought.