I've been using just one lately, but I generally find that I sleep more comfortably with two pillows under my head.
Although it also depends on the softness of the pillows. The reason I use two is to keep my head slightly elevated, so if I have one thick really firm pillow, that tends to do the trick. Likewise, pillows that are ridiculously soft, such as most hotel pillows, which your head just sinks right into, I tend to have a hard time sleeping with. So, for all intents and purposes, given average size and softness, two pillows under my head does the trick.