I suspect as many as then keeps your spine straight at the neck. It'd depend on the depth of the pillows too. You can buy especially shaped ones that are supposed to be good for you.
Memory foam pillows and mattresses don't suit me as I find them to hot and the pillows have been too firm in my experience. You may be able to buy different firmness of pillow but as I favour natural materials I've never looked into it.
I have both tempur 'shaped' and 'normal' pillows, I just pull them about until comfortable. Not had heat issues yet.
"Eek", for 'on front' presumably as either one is trying to breathe through the pillow/matress with a crushed nose, or having to twist one's head/neck around to an uncomfortable side position. Neither seems ideal.