You're looking for at least 10x turnover - meaning if you have a 55g tank, you want a minimum of 550 gallons/hour of flow from HOB
filters, sump returns, and powerheads. For a reef tank with coral, you'll probably want a minimum of 15x, and maybe closer to 20x. How many powerheads it takes depends on the flow rate of the powerheads.
I would use at least two sources of flow for any tank. You want to end up with as chaotic/random flow as you can, and just one flow source isn't going to do that for you. Most folks use at least two powerheads, mounted in the lower back corners, pointing toward the front middle of the tank. The "crossing" pattern of the flows will tend to make a somewhat random current.
Of course, there's always more than one way to accomplish random flow and a lot of it depends on how your rockwork is set up. Bouncing flow off the side glass, or in corners, is also a great way to "randomize" the flow.