"Each document in your
index belongs to a single primary shard, so the number of primary shards that you
have determines the maximum amount of data that your index can hold."
can anyone help me understand this?
If a single shard can hold for example for your use case 1m documents and you have 5 primary shards, you'll be able to index up to 5m documents in your index before it reaches the limits you have.
What if my harddisk is of 5GB, still I would be able to store 5m of data in my index?
It depends. But for a use case I have, I'm indexing around 250 mb for 1m documents.
I think Lucene limits the number of documents per shard to around 2 billion, so shards can get quite big. You are however likely to find that your ideal shard size based on query performance is smaller than that.