I've always had two and I don't intend to add more than 2 if I should ever require more pillows in the future.
That's why I was mentioning sleeping without a pillow because I had read it somewhere that it's actually more beneficial than with pillows because that was the natural state of our body at rest. I haven't tried it at yet but I know a couple of people who swear by it but I'll have to do a bit more research before I try it myself.