how many words should i write?

the longer, the better?

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.

spend a few minutes here and look at some of the essays.

http://www.jiskha.com/search/index.cgi?query=sat+essay

When determining how many words to write, it depends on various factors such as the context, purpose, and audience. There is no universally right answer as it varies from situation to situation. Here are a few considerations to help you decide:

1. Purpose: Determine the purpose of your writing. Are you attempting to provide a concise answer or explore a topic in depth? In some cases, a short and precise response might be more effective, while in others, a longer and more detailed piece might be necessary.

2. Audience: Consider who will be reading your writing. Different audiences have different preferences and expectations. Some may prefer brief and to-the-point information, while others may appreciate a more substantial and comprehensive explanation.

3. Guidelines or Requirements: Take into account any guidelines or requirements provided by your instructor, employer, or any specific writing format. They may specify a word count range that you need to adhere to.

4. Topic Complexity: The complexity of your topic can also influence the length of your writing. A complex subject may require more words to fully explain and explore, whereas a simple topic could be covered adequately with fewer words.

Ultimately, the goal should be to effectively communicate your message or information. Whether it requires a shorter or longer piece will depend on the factors mentioned above. Remember, quality should always take precedence over quantity when it comes to writing.