People are often surprised at finding someone at even a modest gathering who shares a birthday with someone else. Nonetheless, it’s fairly easy to show that only 23 people are needed in a group to have a 50-50 chance of at least one coincidental birthday.
The reason the number is as low as 23 is that you aren’t looking for a specific match. It doesn’t have to be two particular people or a given date. Any match involving any date or any two people is enough to create a coincidence. Indeed, there are 253 different pairings possible among 23 people, any of which could lead to a match.