Shards and replicas are covered in the elasticsearch documentation here:
Shards Per Index
Yes, I'm going to demonstrate replication and reallocation of data in a future post :)
So What Is Ideal?
Well...as some of my SANS instructors were so fond of saying when we would ask questions in class, "that depends". It depends on your index/search ratio, on the number of nodes in your cluster, on the type of data you're indexing, on your requirements for search speed, on your requirements for data replication and various other factors. Your goal should be to test multiple configurations with a defined data set so you can time data import and search responses. Pay attention to disk and other resource utilisation when you're importing and searching data. Don't be afraid to delete an index and start over with different shard/replica ratios!