Sixteen of them are.
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The hexadecimal system.
There are 6 digits in a hexadecimal because the root, hex in math means six. Like hexagon which has 6 sides, so the hexadecimal has 6 digits.
9
8 in octal, 16 in hexadecimal.
Because you need symbols, that occupy only one 'place', for sixteen digits, and we only have ten common numerical digits.