I have one cat and his box can get stinky, even with daily scooping.

This helps: Use a cat litter with carbon. It does make a difference!

Once a month, bleach out the litter box and wash.rinse completely so you don't poison your cat. I put my cat's box in the tub and run the shower, soak in bleach/water.

Also, if your cat's box is very old most likely it has tons of small scratches in the plastic that trap bacteria. If so, toss it and buy a new one.