How to get the replication factor of C* cluster?

In the versions 3.0 + Cassandra you can get the RF details from the system_schema keyspace in the system_schema.keyspaces replication column.

cassandra@cqlsh:system_schema> SELECT * FROM system_schema.keyspaces;

 keyspace_name      | durable_writes | replication
--------------------+----------------+-------------------------------------------------------------------------------------
        system_auth |           True | {'class': 'org.apache.cassandra.locator.SimpleStrategy', 'replication_factor': '1'}
      system_schema |           True |                             {'class': 'org.apache.cassandra.locator.LocalStrategy'}
 system_distributed |           True | {'class': 'org.apache.cassandra.locator.SimpleStrategy', 'replication_factor': '3'}
            company |           True | {'class': 'org.apache.cassandra.locator.SimpleStrategy', 'replication_factor': '2'}
             system |           True |                             {'class': 'org.apache.cassandra.locator.LocalStrategy'}
             jerry  |           True |                   {'class': 'org.apache.cassandra.locator.NetworkTopologyStrategy'}
      system_traces |           True | {'class': 'org.apache.cassandra.locator.SimpleStrategy', 'replication_factor': '2'}

A cluster doesn't have a replication factor, however your keyspaces does.

If you want to look at the replication factor of a given keyspace, simply execute SELECT * FROM system_schema.keyspaces; and it will print all replication information you need.

Consider using DESCRIBE SCHEMA - it's likely that using system.schema_keyspaces will fail to work in a future version (such as 3.0+, where schema is moved to system_schema);