The number of powerheads you need will be determined by what you are planning on keeping in the tank. If just FOWLR then you really dont need much.
A good rule of thumb when looking at how many powerheads are needed is looking at the turnover rate (X).
Turnover Rate (X) = Gallons Per Hour (GPH) / Tank Size (Gallons)
For LPS corals you want to be somewhere around 20X to 25X
For SPS corals you want to be up around 30X
So, to figure out your turnover rate add up all the gallons per hour that your pump and powerheads do. For a 110G tank you will want somewhere around 2400GPH for LPS.
Its not an exact science but it seems to work pretty well.
