Word count is not a good indicator of how well you will do. I did advanced and my essays were usually 1400 words (which is very long) but I scored about the same as people who only wrote 1000 words or even less. It is better to be concise and clear than waffling on and on. However, as a general guide you should aim for at least 750 words (75 + 200 + 200 + 200 + 75) to get a band 5.