There are no limitations to how many times you can take the ACT. But how do you know if you should take it again?  ACT says that you should definitely consider retesting if:
-You had problems during the test, such as misunderstanding the directions or not feeling well.
-You aren't satisfied that your scores accurately represent your abilities.
-You see a discrepancy between your ACT scores and your high school grades.
-You have completed coursework or an intensive review in subject areas included in the ACT
  since you tested.

Students that took the ACT more than once had these results:

-55 percent increased their composite score
-23 percent decreased their composite score
-22 percent did not change their composite score