Taking a stab at it here, Howard! ;)
No one can answer your question ... at least not something you will accept. Truth is, no one can. It's up to you. So here are some references for you to use so you can setup a test of your server:
Since you are testing a production server (one that is in use), strongly suggest testing during off peak hours of usage.
And a warning ... expect issues IF you set the test to a quiz and 1000 users.
When doing the test, suggest one learns a few things about live monitoring. These might require opening multiple terminal sessions (via ssh) first, before running a jmeter test.
In one ssh screen run 'top' ... that shows processes running and which are using the most resources as well as memory of server ... how much free, how much is swap.
Install a thing called 'mytop' ... which is top for MySQL. Run it in another terminal screen when running jmeter testing.
By watching those and seeing when jmeter test begins to fail, etc. one can then determine what is max number of users for given system.
Prior to testing, also suggest installing mysqltuner ... http://mysqltuner.com/ and running it.
Have read that MySQL is most efficient when it can use 80% of the RAM available.
So with MySQL and Apache on the same server, one will need to tweak both configs. NOTE: I've seen in your configs the number 1000 for a couple of settings ... which appear to be guesses. Guesses, without testing the system, could be counter productive. One is actually making the situation worse, not better.
Top will show the number of apache services up and running as well as how much memory is being used by Apache. Just like it will show the mysqld daemon and how much memory it's using. If at all possible the idea is to keep the server/services from using SWAP space ... that means data is written to and read from a temporary area of the Disk to complete whatever which means server response will be slower.
Also, one will never be able to get just Apache and MySQL on a single server to use ALL the memory ... the operating system itself needs to reserve enough memory to ... well ... operate!
Some things to check with the quiz ...
How is the quiz set? To display all questions at once? 1 question at a time? 5 questions at a time?
Makes a difference. Quiz IS, by it's nature, one of the heavier mods of Moodle in server processing. That is a known fact.
Since you've not established a base line when Moodle was first installed and used, then it's a game of catch up ... testing, tweaking, testing, tweaking ... etc.. until one figures out what the server can handle.
Ok, just trying to help here ... so please, don't respond to this with the same question. You've some work to do!
'spirit of sharing', Ken