DOCSP-33786 clarify number of mongosync instances (#211) (#212)

* DOCSP-33786 clarify number of mongosync instances

* DOCSP-33786 clarify number of mongosync instances

* DOCSP-33786 tech edit

Author: ltran-mdb2

source/multiple-mongosyncs.txt

@@ -15,8 +15,8 @@ Use ``mongosync`` on Sharded Clusters
 There are two ways to synchronize :ref:`sharded clusters
 <sharded-cluster>`. You can use either one ``mongosync`` or several
 ``mongosync`` instances. For best performance with large or heavily
-loaded clusters, use multiple ``mongosync`` instances, one
-``mongosync`` for each shard in the cluster.
+loaded clusters, use one ``mongosync`` instance for each shard on the
+source cluster.
 
 .. _c2c-sharded-config-single:
 
@@ -39,6 +39,10 @@ using multiple ``mongosync`` instances.
 Configure Multiple ``mongosync`` Instances
 ------------------------------------------
 
+The number of ``mongosync`` instances must match the number of shards on
+the source cluster. For a replica set source, you can only use one
+``mongosync`` instance. 
+
 To configure multiple ``mongosync`` instances:
 
 #. :ref:`Verify cluster configuration <c2c-shard-config-verify>`