Since JMeter 2.5 there is a new option called “Concurrent pool size”.
Whats so important about this feature?
To answer this lets first understand how browser works and what an end user response time means.
For more information on parallel http connections in a browser go to following link.
Fetching the content of a page using parallel connections dramatically reduces the render time.
Now, in JMeter,
Concurrent pool size is the number of parallel connection that JMeter will open while fetching the resources. if we set this as 4 connections, then it means 4 connections per threads (users). So if the load generator can support 200 user threads, using connection pool option you should reduce the user threads per machine accordingly.