Hello,
As you rightly point out, the true answer here is “it depends”! Though I will say that I agree with the general tone of feedback you are getting.
I wouldn't count the cases you are doing as an interviewer as part of your case “count” - it's a fundamentally different role. So my understanding is that you would then do 15 cases. I would say that 15 full cases is on the lower end, but if you are well-prepared already and understand how the case process works, you would certainly be able to improve quite a bit. I would recommend doing fewer cases as interviewer and more as interviewee, if possible. Working with coaches could be a great way to ensure you are getting the chance to practice + get good feedback, but you could also ask your friends or family to help you practice.
I also agree with everyone who emphasizes the importance of getting some rest, particularly in the week immediately leading up to the interview! Over-casing is a very real issue, so make sure you are taking care of yourself.
My overall advice is to take it day by day. You have three weeks to prepare for the interview, and instead of worrying about what is too much or too little, I would focus on getting in as much good practice in as possible (you as the interviewee, try behavioral questions too, etc.). However many hours you spend, make sure that those hours are effective, that you are not burning out, and that you have sufficient time for rest, schoolwork, and all your other responsibilities.