While comment's suggestion works, I experience a few problems with it. Consequently, I always add a calculation field to my tables, and make it unstored, get( recordNumber).
See attached example file and screenshots. The advantage of using a layout variable is that you don't need to modify the field definitions.
The disadvantage, in my experience, is that any time you touch the formatting of the variable, it resizes itself.
SimpleRecNum.fmp12