Certainly no more than the room holds. 189, no more than 35. You are not obliged to add more than the course maximum indicated on the roster, but you are expected to add up to the maximum if there is student demand for the course. Graduating Seniors have priority. Students from open university, those on academic renewal who are repeating the course should be last on the list.
Some instructors do a lottery.
ONLY with permission from the Chair. Exceptions are granted by the chair with final approval coming from the Dean. The Schedule of Classes lists exam times. Furthermore, counting on ALL students in a tightly packed classroom to avoid looking at a test or answer sheet that may only be located a mere six inches away is also naive. It is fairest to use different tests for different sections, to use multiple versions of the same test in each section, and to actively observe students while they are taking an exam.