When selecting a microphone, a condenser mic is your best choice since most are able to pick up voices from up to 7 feet away and are more capable of a flat wide-range frequency response.  Before positioning your microphones, imagine a 60 degree cone of sensitivity emerging from the front of the mic.  Vocalists within that cone will be picked up louder than those outside the area.  Also keep in mind that it is not possible to mic all voices evenly.  Some voices will be louder than others.  Take this into account when positioning your vocalists. 

High quality mics will generally pick up about 25 people each, therefore use two microphones for up to 50 singers and add one mic for each additional 25. (For example, a group of 58 would need three mics; a group of 80 would need four.)

If more than the recommended number of microphones are used, partial cancellation may occur.  This occurs when the 60 degree cones overlap each other.  A voice that is picked up equally by two microphones will not get heard.

When positioning the microphones over the choir, they should be placed one foot in front of the front row and 24 to 36 inches over the heads of the front row.  The mic should be pointed at the heads of the back row.  This formula is very effective for up to three rows of vocalists.

Microphones may be either set up on mic stands or hung from the ceiling.  Your decision depends on how long the system will be in place and how portable they must remain.  If the system is a temporary set-up or the microphones need to be portable for other applications, mic stands will be your first choice.  If you desire a neat, clean appearance and the system will be in place for a long time, hanging the microphones from the ceiling is the best choice.