I have never heard a rule on the number/size of corals for a given volume of water. Instead, what a person needs to do is plan for the growth of corals, and leave spaces around them. Also, remember that most of the corals that are successfully kept are photosynthetic. Meaning that you have to make sure that corals don't overshadow each other.
From what I understand, corals produce considerably less waste than fish (for the same size, weight, etc.). So having system capacity to handle coral waste is generally not a problem. Overcrowding and overshadowing cause more problems.
Also remember that corals can be aggressive towards each other, and you may need to leave buffer-zones between some corals.