Many colleges and universities use your scores from the ACT or SAT to determine how you perform compared to other students across the nation. Refer to the admission office webpages for all colleges/universities to which you are planning to apply.
There is no set number of times you should take the tests. A good rule of thumb is to take one toward the end of your sophomore year then use those results to help you decide what kind of preparation (if any) you need do to improve the score required for the college, university or scholarships for which you plan to apply.
First, look at ALL of the colleges/universities in which you are interested to see if they prefer one over the other. Some students report liking one test format over the other. The easiest way to find out which one you prefer is to take the Aspire test (pre-ACT) and the PSAT (pre-SAT) during your freshman or sophomore year. These tests are generally less expensive and given during the school-day rather than the ACT/SAT exams which are given on Saturdays. You can sign up for these pre-tests in the Counseling Center during early September each fall.
You can register for the ACT at their website: www.actstudent.org.
You can register for the SAT at: https://collegereadiness.collegeboard.org/sat/register.