Elasticsearch read timeout, seems to have too many shards?

Elastic Stack Elasticsearch
1

I'm using both elasticsearch 1.4.4/2.1.0 with cluster of 5 hosts on AWS. In the config file, I've set index shard num to 10.
So here comes the strange behavior: I created a index everyday, and when there's about 400 shards or more, the whole cluster returns Read Timeout when using Buik insert API. If I delete some indices, the timeout error disappeared. Anyone meets similar problem? This is really a big obstacle for storing more data

2

How much data in the indices?

3

What type of instances are you using? How much heap have you allocated to Elasticsearch? Do you see anything in the Elasticsearch logs?