This is where a decent collection of various mic types comes in handy, so that you can experiment.
Depends on the room ? where you are allowed to place the mic's ? what type of sound is required by the producer ? and more importantly, your budget. To answer this question properly I really need more information. I do have my own preferences, but no two situations are alike.
you could mic every instrument individually if its like a 4 piece orchestra. in which case you should be looking at small or medium side diaphragm condenser microphones with a cardioid or hyper cardioid pickup pattern and a bass roll off (preferable)
if its a large orchestra, you will want 2 or more 'shotgun' condenser microphones. these can be placed 1. on cables, hung from above (see a stageing guy) 2. a few meters infront, on stands, or 3. at the back of the room/hall, depending on what sound you want. 1. suttle. 2. direct. 3. reverberant(be careful if theres a crowd, they make NOISE)
this is assuming you are recording them live, outside, or ANYWHERE but your house or a studio.
in which case if its a small group, you want 2 LARGE diaphragm condenser microphones, with bass roll off. if the preformers wish to play in a circle or semi circle, a mic with a selectable pickup pattern may be useful. try omni, for an alternate sound, also try traditional cardioid in this instance.
if it is a large group in a studio, meduium to large diaphram condensors.
if its a large group in a LARGE studio, add shotgun microphones to the medium to large condensors, to capture the room sound, place them at back of room.