First select the numbers with the selection tool, then right click on it and select "Create Outlines". Now keep the paths selected and go to Object>Path>Offset Path. Now pick how thick you want the outline to be by adjusting the "offset". Miter, bevel, and round changes whether the edges of the offset path are sharp, squared off, or rounded, and the miter limit changes how long and sharp the offset path would be: a good tip is to leave it fairly low around 4 or 5 so you don't have crazy pointy edges all over the place.