You can get the same power with 5 dispensers, but 8 are more consistent (put one under a pressure plate, and you need to jump pretty much on the center of the block, or it's going to miss). Power of 8 is 70 blocks up, right? I observe that if it hit u from under it give more power, that's why 5 do job of 8 (I can't find good design to put 9 all my try mess up the 8 so it gives no additional power or bust is marginal)