For those not familiar, a Replica Set is defined as a redundant configuration of three servers. A Sharded Database is defined as a horizintally scalled database, where each Shard is defined as a Replica Set. Therefore, a sharded Mongo cluster involves a minimum of 11 servers for a 2 shard cluster, and increases by three servers for each additional shard. So, a sharded cluster always has 11, 14, 17, 20, 23, etc server instances. That is, there's 2 shards of 3 servers each, 3 more config controllers, and 2 routers. 11 servers total for a 2 shard cluster.