The answer is 9 000 000 people
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EVERYBODY in the US has the same birthday as someone else in the US. The converse would be that someone in the US has his/her own birthday and nobody else in the entire population of 300,000,000 people in the US was born on that day.
Some people call that your golden birthday.
The probability of two people's birthday being the same is actually more likely than many would think. The key thing is to note that it doesn't matter what the first person's birthday is. All we need to work out is the probability that the second person has a birthday on any specific day. This probability is 1/365.25 The probability that they were born on June 10th is 1/365.25. The probability that they were born on February 2nd is 1/365.25 and the probability that they were born on the same day as you is 1/365.25
On average every day there are 1.9 million people celebrating their birthday. That is the total global population (7 billion) divided by the number of days in a year (365).
No 12 year olds have a birthday every day. They only have a birthday once a year.