"More is better" within reason. Quality trumps quantity. It is more important to get the higher HU (Heat Units) than a whole bunch of lower HU stuff as you would also be adding a lot of solid matterwith quantity.
Over the years seems the consensus on a judicious effective amount round these here parts has been 1/4 tsp of the 90K or higher cayenne per drink. That works out to roughly 3/4 tsp per qt +/-.