For preparation, I recommend 3 three things:
- Read Case Interview Secrets from Victor Cheng. I would almost go as far as to say: you have almost no chance in getting through the day without it. Think about his LOMS (Look over my shoulder) package.
- Leverage the boot camp from PrepLounge to get up to speed on the basics both in terms of structure and content.
- Do as many meetings with fellow interviewees as possible (through the PrepLounge meeting board) and think about investing in a coaching session (Francesco is amazing, e.g.). Ultimately, the “right” number of cases depends on a) your previous knowledge, b) your available time and c) your progress. Note, like preparing for a big competition in sports, there is a plateau for case performance after which the marginal value of doing one more case becomes negative.
Finally, to help you put my recommendation into perspective, I was preparing for several final round interviews. I had done a tone of reading, watched free YouTube videos (Firmsconsulting, MasterTheCase, etc.), talked to people, and booked coaching sessions. I arrived at the conclusion that without decent preparation it's exceptionally difficult to pass. It's very transparent on what the consultancies expect from you so everybody has a fair chance to convince the interviewers. While that is generally good, it means that most interviewees have practiced a lot and, as result raised the average.
Doing what I did, helped me secure a Sen. Associate position at McKinsey. Keep in mind, this is n=1.