I used to go as much as 4, down to 20°F when I was using a thin wind shell: tech base layer, long sleeve jersey, long sleeve t-shirt, and the wind shell.

However now I have a thicker Nashbar soft shell jacket which takes the place of 2 layers. I've ridden as low as 14°F with a tech base layer, sweat shirt, and the soft shell jacket. When it's up into the 30's I'll do the tech base layer, long sleeve t-shirt, and the soft shell jacket. Works great!