Mixing load balancing and availability is a common and simple way to scale-out your servers and databases. This can be done by distributing your data across several shards and mixing this distribution with replication by replicating every shard. Then, you can use the replicas to read queries. This technique works well for a read-heavy application. RavenDB does this out of the box.
You may choose to shard your database with dedicated failover nodes. In this configuration, each shard is configured as the replication master, and in front of each shard, a dedicated server is configured as the replication slave (for each master). In this case, the replication node numbers are at least the same shard nodes. The advantage is that if one of the primary nodes is failing, RavenDB will automatically switch to the replicated copy.
Another option is to use sharding primarily as a means of reducing load on the servers and set up replication between the different nodes without...