I count about 50 (there are a few more in the CS). Let's assume we just take about 15 different colours (basic VGA palette or similar), and 5 different shapes (circle, square, 5-star, 6-star, triangle) for the background, that's 15*5 = 75 different backgrounds. And then we can let it glow less or more depending on the effect magnitude.And that's not even including variations of the item inside the icon itself, of which there can be dozens more.
You missed the most important point, you may be able to make (much more than 75, remember items with multiple effects?) icons, this does not mean they are meaningful, intuitive to the player or even understandable.
Systems which use primitive shape and colour changes only work when there are a limited number of them, their usability can be increased by using in built knowledge, see Civilization 5's unit icons, Shield shaped background means the unit is defending sword icon means swordsmen, bow & arrow means archers.
Pink does not inherently mean dragonslaying, a circle does not mean mana regeneration, these are not concepts built in to our psyches and background knowledge.