What's the minimum amount of case studies you would consider necessary to practise to be successful at the actual interview?
How many case studies should I do?
Quality > Quantity. Focus on:
1. finding the right partner, and / or expert to work with.
2. focusing on your negatives maniacally and not making those mistakes again
3. read through as many generic (tier-2) cases as you can to build a mental database of what to do when, and then try to start solving the tier-1, more difficult cases
4. Typically I see good case solvers once they've done anywhere between 20-40 solid, difficult cases covering a range of industries and business problems