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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With 9 digits, there are a billion combinations, which is about 3 times more than the current US population.
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**.
To answer the wording of your question directly.
The reason no two people have the same order of numbers is because the system responsible for assigning numbers is aware of all numbers that have ever been issued, and will not issue the same number twice.
Social Security numbers aren’t chosen; they’re assigned. The Social Security Administration ensures the numbers are unique.
Currently the US population is less than the number of SSNs available. So they haven’t run into that problem yet. But when they do run into that problem, it’s probably a minor problem compared to the problem of keeping the system funded. (As one of my favorite political websites notes, “I hope Social Security is abolished long before we run out of Social Security Numbers.”)
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