It is best to have one box per cat. Cats also prefer the fine-grained unscented litters (scoopable). Many cats refuse to use covered boxes, but many cats prefer them! Try getting one uncovered box and one that is covered and see which one is used the most!
It really depends on the cats. When I had just 2 cats they shared one box with no problems. Then I upped it to two when we had 3. Now I have 11 and 6 boxes through the house. I guess I am very lucky because every one of them has a hood and nobody has peed or pooped outside the box.