it depends on what the arrangement and production of the song call for.
if it's a solo acoustic singer/songwriter type, i'll almost always use 2 mics and quite often 3. two will be a spaced pair of MXL603's, one on the neck around 12/14th fret, and the other by the player's ear, over their strumming shoulder, pointing down and back towards the guitar. i'll then put my dragonfly or V77 about 4ft out from the guitarist in a place that sounds good and mix that in. the goal here is to make the guitar take up a lot of sonic realestate. i can "get by" fine with only one mic, but IMO it's a lot like listening with only 1 ear.
mic placement of that "over the shoulder" mic will usually change dramatically if the person's singing while they're playing.
if i'm just adding an acoustic track to a song with drums, bass, other guitars, etc, i don't want it to take up as much space. i'll typically mic this with 1 mic and not as closely. usually in this scenario i'm looking more for the percussive aspect of the guitar--pick on strings and such, than a full spectrum acoustic recording.
in the case of an acoustic duo, i'll close mic the guitars and make them a little "smaller" sounding than i would for a soloist, pan one person hard L and the other hard R and throw in a couple room mics to glue it together.
good luck!
cheers,
wade