What is the minimum number that must share the same birthday (month & day) each year, given that one such birthday is February 29? If there are 6,392 students at Stack Exchange College. What is the minimum number that must share the same birthday (month & day) each year, given that one such birthday is February 29?

Now I'm not entirely sure i'm answering your question correctly but just compute 6392/366 and round down to 17. Unless you are asking for the expected value in which case this is incorrect.