You can add nodes to a replica set or sharded cluster instance.
Note You are not allowed to add nodes to standalone instances.
- Replica set instances
By default, a replica set instance has three nodes. For more information, see Change the number of nodes for a replica set instance.
- Sharded cluster instances
By default, a sharded cluster instance has two shards and two mongos. For more information, see Change the configuration of a sharded cluster instance.