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).
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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.
Everyone shares their birthday with someone or another.
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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.