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Nutella
There is no exact number to determine how likely you are to receive a second round offer (as this depends on more than one factor including your own ranking, the number of people who reject the offer, and the 4% rule).
Keep in mind that rejected offers don’t have to come from others at your school. For example, if a school has 350 in their year 8 cohort and 10 receive an offer in the first round (less than 4%), 4 people can still get an offer in the following rounds from rejections at other schools (as long as their ranking qualifies them for this).
Best of luck!