How I can create serial number for category number stored in variable

[Abdul Habib Yousufzai]

I have the following value
1=1
1=2
1=3

2=1
2=2
2=3
2=4

9=1
9=2
9=3
9=4
9=5
9=6
9=7
9=8
9=9
9=10

[Nick Cox]

This may help.

* Example generated by -dataex-. To install: ssc install dataex
clear
input byte whatever
1
1
1
2
2
2
2
9
9
9
9
9
9
9
9
9
9
end

bysort whatever : gen wanted = _n

list, sepby(whatever)

     +-------------------+
     | whatever   wanted |
     |-------------------|
  1. |        1        1 |
  2. |        1        2 |
  3. |        1        3 |
     |-------------------|
  4. |        2        1 |
  5. |        2        2 |
  6. |        2        3 |
  7. |        2        4 |
     |-------------------|
  8. |        9        1 |
  9. |        9        2 |
 10. |        9        3 |
 11. |        9        4 |
 12. |        9        5 |
 13. |        9        6 |
 14. |        9        7 |
 15. |        9        8 |
 16. |        9        9 |
 17. |        9       10 |
     +-------------------+

[Abdul Habib Yousufzai]

Dear Nick Cox,

Thank you so much for your kind assistance. Yousufzai