As FEW times as you can. Although you can register for, and take the MCAT more than once, it looks much better to schools if you can land a good score the first time around.
The MCAT is commonly thought of as the factor that shows how equivalent students from different universities really are, and if you take it too many times, not only will most schools still take your initial sub-par scores into account, but they will also place less emphasis on a good score if you've tried too many times previously (think about it, it's really not much of an accomplishment if you just keep trying over and over again). When I applied, you were even required to ask for a waiver to take the test more than 3 times, simply because that many repeats is a very bad idea for all but very rare situations.
In general, most people take it only once or twice.