Vertical suspensions are usually done with four to six hooks (with extra hooks for the arms in the case of a crucifix suspension), and horizontal suspensions are generally done with eight to twelve hooks, but are not uncommonly done with as many as 24 hooks.The amount of hooks used per suspension depends on two factors, the experience level of suspendee experience and weight of the suspendee. A good rule of thumb is as follows:

15-20 pounds per hook for beginners (ie. 8 hooks for a 140 pound guy)
20-40 pounds per hook for intermediate (ie. 4 hooks for a 140 pound guy)
40-80 pounds per hook for advanced (ie. 2 hooks for a 140 pound guy)
Experienced individuals only should attempt 80+ pounds per hook

As for determining the amount of hooks for each style of suspension, this depends a lot on physical geometry. The vertical suspension really only has one central weight apex, so the least amount of hooks can be used for these. The horizontal suspensions however have four basic areas of weight: chest, hips, legs and arms. Facing upward (in a coma suspension) each point must be held by hooks or by the some other means (such as strapping). Facing downward, (in a superman suspension) the chest and hips are combined into one variable due to the fact that the hips do not bend forward.