Ultimately, your essay will be evaluated on the information you present, not on the number of paragraphs in the essay. Early in your academic life, teachers and lecturers may give you both a structure for your essay and a guideline on how long each part of the essay should be. I have seen essay instructions say how many marks are allocated for each section, and my trick is to take the total word count and allocate a percentage of words to each section based on the percentage of marks you can get for it. After all, if the teacher is allocating 80 marks for content in total and you can see 50% of the mark relates to a certain part of the essay, then 50% of your essay’s words should be devoted to that section.
Never apologize for or otherwise undercut the argument you've made or leave your readers with the sense that "this is just little ol' me talking." Leave your readers with the sense that they've been in the company of someone who knows what he or she is doing. Also, if you promised in the introduction that you were going to cover four points and you covered only two (because you couldn't find enough information or you took too long with the first two or you got tired), don't try to cram those last two points into your final paragraph. The "rush job" will be all too apparent. Instead, revise your introduction or take the time to do justice to these other points.