The ‘5 sentence rule’ is usually just a general guideline for the length and substance of your paragraph. A paragraph would generally consists of everything you have to say about a topic. It could be a single sentence, or it could be 12. Though if you were writing an essay or expository piece, I would try to stay within or beyond 5 sentences per paragraph. Other forms of writing, such as stories or memoirs tend to be more flexible with the amount of sentences per paragraph, so you would probably be fine with less than 5 sentences.
