There is such a thing as pillow overkill - Here are some great rules I found written by Carla Jordan Via Elle Decor where she quotes Heather Kearsley Wolf, designer and president of Kearsley, International, Ltd. on the Pillow Do's and Don'ts
On a twin bed, instead of a standard pillow, add one king pillow because it spans the width of the bed; and one to two accent pillows.
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For a full size bed, Wolf recommends two standard pillows, two standard or Euro shams and one to two accent pillows.
Queen beds can easily hold two queen pillows and two queen shams, plus two to three accent pillows or a single bolster.
On king beds, a good combination is three standard pillows and two king shams, plus three smaller accent pillows or three standard pillows with three Euros.