These are very common questions, and they have a consistent answer. A Retake on How Many Times to Take the SAT or ACT
My advice to applicants is to figure out what your target college is, and to take a look at the score ranges of the most recently reported admitted class. College Search - Harvard College: SAT®, AP®, CLEP®
From there, figure out what score you should aim for. (I recommend trying to get to or above the 75th percentile of the admitted class at the target college on each test section, knowing that not everyone will get to that level.) Then READ, READ, READ, and READ any kinds of books or magazines you like. (That's the best single form of preparation for all standardized tests: practice reading so that you read accurately and rapidly.) Be familiar with the test format through practicing with a genuine previous test under actual test time limits before you take the test "for real." Prepare well (get a good night's sleep) each and every time you take a standardized test. Quit when you reach your target score, or when you reach the highest score you reasonably think you can, and then devote yourself to other activities besides test-taking. Always take the test honestly according to the test rules. Never cheat. Don't forget to consider How do top scorers on tests fail to gain admission to top schools?
And most of all, relax. Most people do better on tests if they treat them as interesting challenges (which they are) and not as life-and-death experiences (which they are not).
Good luck. Does that answer your question?