You can use Insert Sequence Numbers plug-in (32-bit)

or a .jsee-macro like this one:

var n = document.GetLines();
var a = new Array();
for (var i=1; i<(n+1); i++) a.push(i + ") " + document.GetLine(i));
document.selection.SelectAll();
document.write(a.join("\n"));
Quit();