A number of
marimbas, usually over 20, all playing one piece of music. The
orchestration for marimba choir is often an exact
translation from string oprchestration, since very few pieces if any are actually written for marimba choir.
The largest marimba choir ever was April 2, 1996 at the
University of North Texas. Myself and 61 other people all piled onto a stage at UNT and played an arrangement of
Samuel Barber's
Adagio For Strings with 58 marimbas and four
bass marimbas. The orchestration was in ten parts, featuring
Violin I(a and b) and II(a and b),
Viola I(a and b) and II, and
Cello I and II (a and b).
Maybe I'm biased but a marimba is one of the most beautiful instruments I've ever heard, and that many of them playing like strings sounded like a choir of
angels. Go see a marimba choir if you ever get the chance.