Now, this depends on the discipline, and the journal. It is why you need to choose a journal (and preferable two back ups) very early in the writing process.
In general, 5,000 words seems to be the "sweet spot"- long enough for most journals, and rarely too long for all but the most parsimonious publications.
Don't forget the brief note sections of journals, which I explore another time. Remember, shorter articles maximize your ability to publish- your can finish one article and get to the next one. 12, 000 essays are often far too verbose, except for the rarest of works.