We have 10 numbers, 0-9, precisely because we count in base 10.
If we counted in base 19, we would have 19 numbers.
When doing hexadecimal, we have 16 numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, and f.
When doing octal, we have 8 numbers: 0, 1, 2, 3, 4, 5, 6, and 7.
When doing binary, we only have 2 numbers: 0 and 1.