The model regarding paragraph length that your teacher undoubtedly taught you involves a topic sentence, a number of facts that support that core idea, and a concluding sentence. The proviso about the number of sentences between the topic sentence and the conclusion was not given to you because it was the magic formula for creating paragraphs of the perfect length; rather, your educator was attempting to give you a good reason to do adequate research on your topic. Academic writing yields the best examples of the topic-support-conclusion paragraph structure.
You can see from this example how a topic is introduced, supported, and then brought to its natural conclusion. Yet, not all writing is academic, and once you have learned the concept behind good paragraph construction—which is really the art of focused writing in disguise—you should know that there are times when paragraph “rules” can, and should, be broken.