How many runs are necessary to get useful results?
The answer to this question depends on the following:
For factorial designs, finding effects is the goal. An effect is how much a response changes when factor levels are changed. Power is the probability of detecting significant effects.
For response surface designs the goal is generally optimization. Having a precise model is important to making good predictions for finding the optimum factor combinations. Fraction of Design Space (FDS) is the proportion of the design where the precision of the estimate is better than the acceptable margin of error.