What is the highest number of threads that is reasonable to simultaneously run in Jmeter?

JMeter can simulate a very High Load provided you use it right.

Don't listen to Urban Legends that say JMeter cannot handle high load.

Now as for answer, it depends on:

• your machine power

• your jvm 32 bits or 64 bits

• your jvm allocated memory -Xmx

• your test plan ( lot of beanshell, post processor, xpath ... Means lots of cpu)

• your os configuration (tunable)

• Gui / non gui mode

So there is no theorical answer but following Best Practices will ensure JMeter performs well.

Note that with jmeter you can distribute load through remote testing, read:

• Remote Testing > 15.4 Using a different sample sender

And finally use cloud based testing if it's not enough.

Read this for tuning tips:

• http://www.ubik-ingenierie.com/blog/jmeter_performance_tuning_tips/

Read this book for doing load testing and using JMeter correctly.

The JMeter Wiki reports cases where JMeter was used with as much as 1000 threads. I have used it with at most 100 threads, but the Links in the Wiki suggest resource reductions I never tried.