From BoxedIce experience:
MongoDB supports the concept of replica sets which is like master/slave, but any member can become master automatically if the current master fails. It allows for automated, near instant failover. We’re using it live in production and each of our database shards has 4 members + 1 arbiter. MongoDB has a limit of 7 members max, but why would ever need that many? Why do we need 4 copies of our data?
Data redundancy: sometimes it is not enough to just use multiple data centers for availability. Multi rack redundancy can be helpful too.
Original title and link: MongoDB: When would you need more than 7 replica set members? (NoSQL databases © myNoSQL)