A lot of interviews are conversational, so you can throw in questions at any point. That shows the interviewer you’re curious and conversational. I tried to ask questions ABOUT THE INTERVIEWER when I made my rounds. People love talking about themselves, so ask about hobbies, interests, research they’ve done, etc.
If the interview vibe isn’t conversational, 2 or 3 good questions is probably good at the end. I would usually include the question: “what qualities is the admissions committee at [insert dental school] looking for in an applicant?”... then when they tell you, try to let them know how you could add unique value relating to those qualities.