The birthday paradox or birthday problem is one of the old mathematical puzzles. The most common version of the problem is: how many people must be present in order to have more than a 50% probability that at least two people have the same birthday?
The number of people after which it is more likely that two people have the same birthday than that everyone has a different birthday is surprisingly small and deviates from the image that many people have.
With the formula below, it can be shown that when 23 people are present, it is more likely that two people have the same birthday than that everyone has a different birthday.
Probabilities graphically
The horizontal axis of the graph shows the number of people and the vertical axis the probability that at least two people have the same birthday.
You can find more information about the birthday problem and how to show it more precisely: https://mathworld.wolfram.com/BirthdayProblem.html