I have the Audio Technica 4050 that someone else mentioned. It's a good microphone, and offers the choise of cardioid, omni, and figure-eight pickup patterns. I use it mainly for recording.

To do the "one mic" routine, you need an omnidirectional pickup pattern. Generally a condenser mic will be best, on a shockmount so you don't pick up the rumble of all those moving feet as people move in and out of the microphone's range. You should be aware that the sound man has essentially NO control in this setup, and that the mic is likely to pick up some ambient noise that nobody will be able to do anything about. I suspect that in a lot of venues it would be impractical.

A lot (most) of the old bluegrass bands used this approach until the venues began to get bigger and we all got more technically sophisticated back in the late 60's. Steve Earle and the Del McCoury band have gone back to this setup recently in some of their shows; my guess is that the "retro" aspect is part of the appeal. I understand they found the choreography to be quite a challenge initially -- six guys holding instruments with long necks, constantly jockeying for position and trying to avoid bumping into each other. Don't know how the sonic blend works out; all of us have higher expectations regarding sound than we used to a few decades ago. I'll bet it's fun to watch, though.