Regardless of what base you're using, 10 is always the nth number. In base 10 (normal numbers), 10 is 10th. In base 2 it is the 2nd.

1. 1
2. 10
3. 11

In base 16 (hexadecimal) it is the 16th.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6
7. 7
8. 8
9. 9
10. A
11. B
12. C
13. D
14. E
15. F
16. 10

The original joke is "there are 10 kinds of people, those who understand binary and those who don't l" because 10 in binary is 2 in base 10. But they're pointing out that a similar joke works for all bases of numbers.