In a sharded environment, the output of the "mapreduce" command is confusing.

Following is a map-reduce that generates a "tags" collection. That "tags" collection contains 5077 documents.

Problems:
#1: There is no indication of 5077 anywhere in that output.
#2: Based on the groups question, it looks like the output value from each shard may not be the last reduction for that node.
So the output value for a node may misrepresent that data that is actually on that node.

======
> db.runCommand(

{ mapreduce: 'test', map:map, reduce:reduce, out : 'tags'}

);
{
"result" : "tags",
"shardCounts" : {
"localhost:6900" :

{ "input" : 36443, "emit" : 30756, "output" : 2906 }

,
"localhost:6901" :

{ "input" : 18808, "emit" : 12933, "output" : 1948 }

,
"localhost:6902" :

{ "input" : 27274, "emit" : 24070, "output" : 2765 }

,
"localhost:6903" :

{ "input" : 45220, "emit" : 43915, "output" : 3742 }

,
"localhost:6904" :

{ "input" : 48399, "emit" : 45688, "output" : 3842 }

,
"localhost:6905" :

{ "input" : 27413, "emit" : 23050, "output" : 2837 }

},
"counts" :

{ "emit" : NumberLong(180412), "input" : NumberLong(203557), "output" : NumberLong(18040) }

,
"ok" : 1,
"timeMillis" : 10978,
"timing" :

{ "shards" : 10609, "final" : 368 }

}
> db.tags.count()
5077
======

Associated groups question:
http://groups.google.com/group/mongodb-user/browse_thread/thread/65869b34e4230a3b#