You should only take 1 MCAT, and you should take it once you've finished the pre-reqs and prepared enough that you are scoring in your goal range on practice tests.
In addition to Mehd School's advice... If you don't plan on taking a gap year off, then an ideal MCAT test date would be before May of your junior year. That will leave you enough time to get your scores back for applications, which start in June.
You should only take 1 MCAT, and you should take it once you've finished the pre-reqs and prepared enough that you are scoring in your goal range on practice tests.
Sorry, but what is it with people calling the MCAT "the MCATs"?! It's just one test. Singular. Do people say the LSATs? Do people who smoke a lot of pots say "Hey, how 'bout those MCATs? " Figure it out, people. *Drops mic*
... *Picks mic back up* That rant aside, do what Mehd School said. Don't take it until you've taken the prerequisites, and do well in those classes. Don't just get a good grade, but really learn the material so you don't have to take multiple *MCATs*, just one MCAT. You have to learn it anyway, so you might as well make sure you learn it right the first time; relearning things sucks.