Getting the current row number?

What do you mean by “output”? You can do:

:echo line(".") + 1

To display the current line number plus 1. You can bind a keystroke with map, eg:

:noremap <F1> :echo line(".") + 1<cr>

To actually insert the data into the buffer:

:noremap <F1> :execute "normal! i" . ( line(".") + 1 )<cr>