If you are counting words, you are better not taking the exam, that is so 9th grade thinking.
You are given a prompt to respond to on the SAT. In 30 min, you hand in the paper. I recommend of that 30 min, you spend 5-6 min making an outline and organizing the paper.
Papers usually are three to four paragraphs long. You need and opening statement, the body, which is one to three paragraphs, and a strong concluding paragraph.
It is NOT graded on length, but did it answer the question presented.
Your question deeply troubles me, as it is apparent somehow you have avoided teachers who are preparing you for essay writing, and the OTHER parts of the SAT.
I strongly recommend you visit the local college bookstore, or BarnesNoble, and purchase a SAT prepration book, there are several good ones, and start for yourself.