Country specific fields in database schema
You could deal with such scenario by using Descriptive Metadata tables, have a look at the Wordpress database diagram.

As you can see it makes use of three metadata tables that can store any number of parameter per Post, Comment and User.

So in your case a bare-bones implementation could work like:

----------------
|     User     |
----------------
|uid| uname    |
----------------
| 1 | UserOne  |
| 2 | UserTwo  |
| 3 | UserThree|
----------------

----------------
|   MetaData   |
----------------
|mid |  value  |
----------------
|  1 |   TIN   |
|  2 |   SSN   |
|  3 |   UID   |
----------------

------------------------
|      UserMeta        |
------------------------
| id|uid|mid| value    |
------------------------
| 1 | 1 | 1 | 123131   |
| 2 | 1 | 2 | 343234-F |
| 3 | 2 | 1 | 123654   |
| 4 | 3 | 3 | OINASD-01|
| 5 | 3 | 1 | 123984   |
------------------------