If you are new to the Nashville Number System you can find detailed information at wikipedia or have a look some of the videos at YouTube.
See these examples:
Within Chord Sheet Maker Online you do not actually write the numbers, but can transform the resulting chart to show numbers instead of regular letters.
There are three steps required to make a Nashville Number Chart ...
See some more nns examples.
In the settings you can choose your preferred display, i.e. whether you prefer Arabic or Roman numerals. And how minor chords should be represented.