In terms of the number of paragraphs, there's no hard and fast rule. Personally, I preferred to have 3 body paragraphs, since I tended to frame my response as three broad ideas, with a number of smaller 'sub-points' within each of them. As a result, each of those paragraphs was quite fleshed out and of considerable length. However, I know many high-achieving students prefer having 4 paragraphs - if you find it easier to group your response this way, then it's just as good a method.
In terms of having a 'balanced' discussion, you're not obligated to explore both sides of the topic equally. In fact, it's important that you develop some sort of contention; despite the topic being worded 'discuss', your 3/4 main points should link together to form some sort of main argument. This contention can be a 'balanced' one (taking a middle stance), or you can 'mostly agree' or 'mostly disagree' with the topic. The most important thing to do here is to make it clear which parts of the topic you agree/disagree with, essentially you shouldn't have two paragraphs that contradict each other.
Hope that helps
