I usually consider 5 couple a minimum for a regular contra dance, but 
triples like Money Musk require more people.
A caller up here, Byron Ricker, has been writing a few dances that he 
calls "quad beckets."  Quad becket dances are becket dances that can be 
danced with 4 or more couples and that never leaves anyone out (unless 
there are an odd number of couples). Many, but not all becket dances 
work this way.  With just 4 couples left, I've sometimes called a quad 
becket with some success.
Rich.