It's hard to say how many drafts you should go through of any essay, but you can generally plan on going through more than one. For a college essay, which will have an impact on your college admission, its especially important to draft and revise as much as you can.
But how many drafts? I think that depends on how many people you have reviewing your essay and how much feedback you get. And yes, you should definitely have others review it.
I'd suggest choosing 3 people that you trust look at your essay. You don't know how many people will be reading your application and they may all have different perspectives, which is why it's important to get several viewpoints. Teachers, parents, a guidance counselor, or a peer that you know is a strong writer (not just your best friend) are all good people to ask to review your work. Be sure to have them read for:
1) Clarity - Does your essay make sense?
2) Flow - Does anything feel abrupt or out of place?
3) Grammar and spelling - Did you miss a comma?
4) Word choice - You want to show your vocabulary, but don't get carried away with SAT words.
You can either have everyone review it at the same time and try to incorporate the comments you think are most helpful (it's good to have advice, but you sometimes you really know best) or it may be easier to have one person read the essay, revise it, then give it to someone else to look at. Do what is easiest for you.
After you've considered everyone's comments and made your changes, give it another read for grammar and, as long as you feel it's your best, there's not much more you can do. Good luck!