Generate Auto increment number of repeating section infopath

1. Open your infopath form.
2. Add an column named is Sr. No in your repeating table/section.
3. Add a textbox control in the column.
4. Right click on textbox say Textbox Properties.
5. Simply add the following formula to the Default Value field,

count(../preceding-sibling::*[local-name() = "YourRepeatingGroupName"])

OR

count(../preceding-sibling::*) + 1