You need enough business knowledge to understand common commercial concepts, not an MBA's worth of memorized theory.

Case interviews usually reward structured reasoning with the information provided. Basic familiarity with profit, growth, pricing, customers, competition and operations is enough to start.

The bigger risk for PhDs is sometimes overcomplicating a simple business question because the topic feels unfamiliar.

Learn the core vocabulary, then build intuition through cases rather than trying to study every industry first. For more detail, see the case structure vs. case framework guide.

imo Counting cases was not useful for me after a point. Tracking repeated mistakes was.

imo You do not need to know every industry. Basic commercial logic goes a long way.