https://en.wikipedia.org/wiki/Birthday_problem
In probability theory, the birthday problem or birthday paradox concerns the probability that, in a set of n randomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are only 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people.
There are scripts with demonstration that paradox. It checks that the 23 people given the likelihood of more than 50% to match the birthday. Problem resolved in Python, JavaScript and C.