Jmeter is probably the best option for you.
If you want the poor man's solution for determining the number of concurrent users, I would recommend checking out this post I put a while back. https://moodle.org/mod/forum/discuss.php?d=313489#p1255346
The only thing I could not get was the built in Jmeter test plan and test course tool in Moodle was the quizzes. I am pretty sure you can adapt the process for the previous post to test quizzes as well.