I believe it is possible to create a load of 50,000 users. I suggest you use several distributed environments each one running no more than 8,000 users. This also depends heavily on your test script.
I am running a JMeter in the Cloud service hosted at Amazon. I already run tests of 20,000 users. So far, I had no reason to run larger tests, however I am pretty sure it is possible.
The JMeter in the Cloud service enables you access to an already pre-configure, unlimited, distributed environment. You can upload your scripts and run your tests. It's immediate and there is not setup involved.
If you wish I can assist you in building such testing environment.