I'm a bit confused.
What do you mean by
decorate? I assume that you mean with food.
Fancy cute looking bowls are fine providing you look at it from the cats point of view and not simply yours. I would suggest that you put practicality and safety first and cosmetic appeal a firm second.
Personally I don't like aluminum/metal and, especially, plastic. My reasons? plastic can harbour all sorts of germs when scratched and I don't like aluminum for humans let alone animals because there is a possible link to Alzheimer's Disease which is being researched. Ceramic bowls scrub up more easily and chips are more easy to detect.
How many? We have 4 dishes per cat which at first glance is a bit excessive I grant you. However, we have a one feline walking demolition unit called Barney who manages to break quite a few.
Under normal circumstances I don't think that 2 per cat is unreasonable. Ideally, Nobody wants to put out food in dirty bowls, especially in summer with flies bringing germs, etc.
We also take our cats' hygiene as seriously as our own with Hibiscrub around the house mainly because of Beanie's medical condition.