Mckinsey FT 2023 only one round of interviews?

I went through this last year for junior internship recruiting. 

Not to be a dick but just in hopes of alleviating your worries a bit, it’s not worth your time thinking about “average success rate” cause with all interviews it’s not a random distribution of who gets an offer and who doesn’t. You have control of how well you perform based on your previous preparation, so sure, think about it a bit if you want, but it helps you very little in the short and long run. 
 

To properly answer your question though, I’ve heard numbers thrown around from anywhere between 25-60% of those who get into decision rounds actually pass their interviews. Because this is a power round format and they’re not filtering out those who actually have no idea what they’re doing through first rounds, I’d expect it to be a bit lower. However, the bar you need to pass is objective, the only person you’re competing against is yourself, and it’s up to you to push past that and hit the bar. Candidates are not graded on a curve against each other with a particular number to pick in mind. If they finish off their recruiting numbers, they’ll just stop extending interviews out to new folks, not shut out the people they already invited to the process.

From my n=1 though, you should probably be expecting to talk to more senior partners - my interview consisted of 2 associate partners, 1 partner, and 1 senior partner. 
 

Best of luck, and buckle up. Don’t forget about your PEI