We have some new clustering docs available here that should be useful: http://druid.io/docs/0.9.0/tutorials/cluster.html
To get HA you need 2x Coordinator, 2x Overlord, 2x+ Broker, 2x+ Historical, and 2x+ MiddleManager. You don’t get much benefit from adding more Coordinators and Overlords (they are failover-based HA) but you do get scaling benefit from adding more Brokers, Historicals, and MiddleManagers.
This doesn’t mean you need 10 machines. Especially for smaller clusters it is very common to colocate Druid services on the same physical machines. You could in theory get by with 2 physical machines although most people do 4–6 for a basic cluster (separating data-heavy services from coordination services).
Also you generally don’t need both Overlord and Realtime nodes. Generally you pick one or the other (we recommend Overlords these days)