500 words is the recommended limit but there is no strict word limit. However, the question is, do you need more than 500 words? The Washington Post recently reprinted an excellent article by Jon Reider that argues that you don’t need more than 500 words to write a strong essay.
Your goal is to write the best essay you can. Although there may be exceptions, I agree that in the majority of cases, you should limit your essay to less than 500 words. Good writing can, and usually should be, succinct.
If your first draft is over 500 words, don’t worry. That is one of the purposes of the editing process.