How many threads does JMeter support?
This depends on the test plan and the number of listeners, but users have reported successfully runnin 1000 threads in a single JMeter instance.
See for example this e-mail thread: http://markmail.org/message/7hqxvchxlt755qb7
See also JMeter best practices: http://jakarta.apache.org/jmeter/usermanual/best-practices.html#lean_mean