Hi,
Please Check this link
http://www.infopathdev.com/howto/tutorials/default.aspx?i=d663d7df90e94e65abb026
32966e8127

Hope this helps.

The position function returns the position of the current XML node in the
current context.  If you are using the postion function on a node that
doesn't repeat, it will always return 1. The position function is most
useful for repeating items.

Try this.  Create a repeating table.  Replace the Text Box in the first cell
with an Expression box. Then, set the value of the Expression Box to
position() using the formula dialog.  Preview the form and insert some rows.
You'll see that the number increments each time you insert a row.

If you need more information. Let me know.

- Scott