If you are poor, 10 - If you are wealthy, 50. (including me)
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If there are 300,000,000 people in the US, then on average you share your birthday with over 82,000 people. Of course, if your birthday is 2/29, you would share it with about 1/4th of that number.
For the chance to be at least 50% that two people share the same birthday, there needs to be 22 people. For the chance to be exactly 100% that two people share the same birthday, there needs to be 366 people. If there was 365 people, there would be a very small chance that each person in the room would have different birthdays. With 366 people, there are not enough individual days for every person to have a different birthday, so there has to be at least one pair.
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.
On the day of your birthday in 2010 you would be 21 years old. Up to that day you would still considered to be 20.
You have to be a EXCELLANT matamatician to figure that out or travel the world and ask people but my guess would be about a few thousands or one million