It depends on many things. For a drumset, you might use 6 mics or more. For a trumpet, you might only use one. It all depends on the amount of controll you want at mixdown time, the amount of tracks and mics you have available, the sound you want, etc...
Just watch out for phase problems when using more than 1 mic. Search these boards for "phase" to find out more. It's talked about everywhere.