With 1 server, I would suggest 1 shard and 0 replicas. The obvious downside of only 1 server is that if it fails you can loose data.
The 8g out of 16g seems about right.
Could you give me different between elasticsearch_shards and replicas ?
An index is composed of shards, and replicas are copies of those shards. Shards are how a single index can live across multiple nodes (e.g. if you had 5 servers, with 5 shards, each server would get 1 shard), since you only have 1 node it doesn't make much sense to have more then 1 shard. You also don't want copies for a single node since there is no where for the copy to go.