MongoDB Documentation, Release 3.0.4 Maximum Number of Voting Members A replica set can have up to 50 members , but only 7 voting members . 6 If the replica set already has 7 voting members, additional members must be non-voting members (page 583). For example, the following 9 member replica set has 7 voting members and 2 non-voting members. See Non-Voting Members (page 583) for more information. Location of the Members A majority of the replica set’s members should be in your application’s main data center. For example, the following 5 member replica set has the majority, 3, of its members in its main data center, Data Center 1 . Electability of Members Some members of the replica set, such as members that have networking restraint or limited resources, should not be able to become primary in a failover . Configure members that should not become primary to have priority 0 (page 567). For example, the secondary member in the third data center with a priority of 0 cannot become primary: 6 While replica sets are the recommended solution for production, a replica set can support up to 50 members in total. If your deployment requires more than 50 members, you’ll need to use master-slave (page 596) replication. However, master-slave replication lacks the automatic failover capabilities. 577 9.2. Replication Concepts