How to determine the number of thread for each user in Jetty

When I look into Jetty. I see this sentence and as a newbie for Jetty.

Load generators should be written in asynchronous programming style, so that limited threads does not limit the maximum number of users that can be simulated. If the generator is not asynchronous, then a thread pool of 2000 may only be able to simulate 500 or less users. The Jetty HttpClient is an ideal basis for building a load generator, as it is asynchronous and can be used to simulate many thousands of connections (see the Cometd Load Tester for a good example of a realistic load generator).

I wonder how to determine the How to determine the number of thread for each user in Jetty.

Since I don't know how to test and which tool should I use. Any hint will be appreciated.

Please.