eli5: with social security numbers you only have 9 digits to choose from and only so many number orders to choose from. How do no 2 people have the same order of numbers?
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9 digits means you have a total pool of 1 billion possible numbers to choose from (although it is a bit less than that because some ranges are not used). The population of the United States is currently about 330 million, and somewhere around 120-150 million additional Americans have died since the program was introduced in the 1930s. This means that only around half of the possible numbers have been used.
Wikipedia cites
> The Social Security Administration does not reuse Social Security numbers. It has issued over 450 million since the start of the program, and at a use rate of about 5.5 million per year. **It says it has enough to last several generations without reuse or changing the number of digits**.
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