Right now basic design the app attacks the web server with 10+ concurrent users, I'd like to control this parameter but can't find it?
I'm not sure to understand your question. The number of concurrent users for a test is setup in the load policy when you configure your test.
Worse scenario you could get less users than expected but i can't a case where you will get more users that the one you configured yourself in the Runtime section.
Could you provide more information about your issue?