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This depends on the test plan and the number of listeners, but users have reported successfully runnin running 1000 threads in a single JMeter instance.
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See also JMeter best practices: http://jakarta.apache.org/jmeter/usermanual/best-practices.html#lean_mean
Also, note that JMeter can place a much higher load on a system than a human user can, as it can make requests much faster. Suppose an average user waits 10 seconds between clicking links. With a wait time of 1 second, JMeter could generate 10 times the load.
So a single JMeter thread can be the equivalent of 10 or more users, at least in terms of number of requests per second.