1 piece of evidence that adds insight per mark is enough.
Simple answer would just be, as much as you can whilst allocating about 2 min per mark. Usually for 3-4 markers you can 'gain' time that you can then allocate to the 5-6 markers, so for these go slightly under the 2 min per mark. If you can write like 30 words in a minute, then for your 6 markers, give yourself ≈15 mins. Aim for about 330 words, give yourself like 2 mins to read the extract (less time if its an image obviously) and about an extra 2 minutes to express your ideas intelligently whilst you're writing. Try churning it out asap, but, most importantly, keeping your response making sense, and providing something insightful. Its easy to get lost in just recounting things from the passage or image. These are very rough guidelines, just a scaffold to use maybe.
You also have to consider that if 2 people were to have the same length responses (say 1.5 pages for a 6-7 marker, which I think would suffice), people obviously have different writing sizes which means that they may be a big difference in the number of words despite the same length answer. That's why I've never been for asking people how many pages they wrote for their essays, because I've seen people with the smallest writing only get 4 pages, while people with quite chunky writing will write 8 pages. People tend to correlate more pages with a better essay, when in reality, that often isn't the case.