the value of the 'virtual user' column in the test definition for a particular user group defines the total number of concurrent users the engine will throw at the server at a given time.
however, to make sure that the test sustains this number of concurrent users throughout the duration of a test, you'll need to set an interval of time and enable the 'continuous load' option on the next tab over.
also, virtual users is not the same as concurrent requests/responses, since a given instance of a virtual user may spawn multiple concurrent connections to the server, based on the recorded traffic pattern in your scenario.
