How many threads does JMeter support?

This depends on the test plan and the number of listeners, but users have reported successfully running 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://jmeter.apache.org/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.