Hi Eman,
MoodleCloud only limits the number of user accounts, so all your users can take the quiz at the same time. In a free site, accounting for one teacher (which in such a case would probably be the admin account), this means you can have 49 students taking a quiz at once.
(For completeness, I'll add that the teacher could do it too, since that account can also be enrolled in the course as a student, but I'm guessing that's not what you're after.)
Mathieu