The contribution elasticsearch-audit-index-config.xml.nxftl does not provide a settings section to set the number of shards and replicas of the audit index.
The default setting is taken from the embedded mode (1shard no replica).
This should be changed to use the same replicas and shards than the document index.
To change the number of replicas to 1 for an existing audit index you can use this:
Changing the number of shard require to create another index with the proper number of shard and migrade the docs from one index to the new one.
See the https://doc.nuxeo.com/display/ADMINDOC/Elasticsearch+setup#ElasticsearchSetup-UpdatingtheAuditLogsIndexConfiguration documentation.
Elasticsearch audit index is now configured with the same number of shards and replicas than the repository index.