As you might remember from our introduction Riak already does a lot of replication for you! It is dependent on the n_val setting in the configuration file to determine how many copies of the data it makes across its adjacent ring segments.
Additionally you can configure this down the bucket type level. For instance you could specific that all buckets of type "map" must have an n_val of 6 (replicated across 6 adjacent nodes). Can be done the following way
riak-admin bucket-type create n_val_of_2 '{"props":{"n_val":2}}'
riak-admin bucket-type activate n_val_of_2
Special Note: You may increase an n_val but you should never decrease an n_val for already written data because this will cause dead data to be persisted across some nodes due to the way Riak handles indexing. Source: http://docs.basho.com/riak/kv/2.2.3/learn/concepts/replication/#setting-the-n-value-n-val
The default value for this is 3 adjacent nodes.
This is a premium addon on the defunt webpages of basho. Documentation to be determined.
We've covered this setup in our configuration introduction but Riak recommends you add additional nodes whenever you will either run out of space at current usage within 10 days or when your total capacity reaches 80% of usage.
Clustering also always add additional space and speed.
Sources: http://docs.basho.com/riak/kv/2.2.3/learn/concepts/replication/