There's no ratio per day. I've sometimes interviewed 8 people in a day and offered them all places, I've also interviewed 8 people in a day and offered no places. If you've got as far as an interview you can assume they're very interested in you (I interview only about 35/40% of those whose applications I have read). It's all about whether you can convince them on the day that you've got what they're looking for. Good luck.