[SamBam]

You need 253 people in the room to have > 50% probability that one will have the same birthday as you.

1 - ((365 - 1)/365)^253 = 0.5005

[uncoder0]

If birthdays are assumed to be distributed symmetrically. If you were born in august you'd need ~12 fewer people and for April you'd need ~12 more.[1]

Not sure how that looks for the worlds population probably averages out pretty well regardless.

[1]: https://www.panix.com/~murphy/bday.html