If I want to write 250-words essay, how can I divide the number of words within the following sections?
There's no correct answer.
Write the essay first, then decide how much space you need to introduce your main topics and how much space you need to state your conclusion.
Obviously, the body of the essay will take up the most space. In my essays, I like to give the reader a good idea about what I am going to talk about, in the introduction; then I talk about it; and then wrap it up with a succinct and relevant conclusion. So I'd use something like the first choice. But styles vary.