I'll let you follow her easy-to-follow tutorial if you'd like to number the comments on your own Blogger blog. The only thing I would add is that I was momentarily stumped at the first instruction of "Go to Template Designer". It turns out that Jenelle had already thoroughly explained that in her post "Layouts and Templates in Blogger" that is part of her awesome "Tech Help for Bloggers" series. In case you just want a quick visual, I'm going to give you a screen capture to show where that is.
Select "Template" from the left hand menu and then click "Customize" and you'll be brought to the Template Designer.
Click "Advanced" and it will open a new menu for you.
Scroll to the very bottom of this menu and you'll find "Add CSS". Ta da! See Jenelle's post for the code you'll need to paste into the box.
Enjoy your numbered comments!