Powerheads are rated for flow, in gallons per hour. You should add up the total amount of flow in your tank -- so if you have two 1,000 gph powerheads, that's 2,000 gph flow total.
You want to the total amount of flow to be at least 10 times, but ideally 20+ times the size of your tank.
So if you have a 60 gallon tank, you want at least 600 to 1,200 gph of flow, or more.
In my old 10 gallon tank, I had 100 times turnover. So more is usually better. In my 90, I had 60 times turnover. They say "If your sand is staying on the bottom, you don't have too much flow."
Most people don't have enough flow in their tanks, IMO. In a 60 gallon I'd want at least two, probably three powerheads total. They should be placed in opposite areas of the tank, facing upwards to the water's surface so that they create a ripple on the surface of the water.