Last year it was 16 (out of 60) for bronze, which is the lowest award - that's pretty much only a quarter of marks needed! Don't worry i just did a past paper, and some of the questions were actually straightforward like considering it was a challenge set by Cambridge University I didn't think those types of questions would come up but they did. The paper is designed to be accessible for most people if you have good AS chemistry knowledge. You're not expected to know everything in that paper, just make educated guesses if you have no idea. I would recommend having sound knowledge of organic chemistry though because that always comes up.