You would need 13. Since there are 12 months, if you gather 12 people and they each have different months, there would be 1 person per month. If you add 1 more person, it is safe to assume that that person will be with another person in a certain month.
When 23 people<span> are gathered, there is more chance than not that 2 of them have the same birthday. In order to help understand this, you should consider how many PAIRS of people there are. With </span>23 people<span> there are 253 different pairings i.e. more than half of the 365 possible birthdays.</span>
