the number and placement of PHs is up to you. assuming you have a fish only tank, you should aim for ~10 X tank volume for water flow. (so 450 gph
there are a few PHs you can get to handle that. Maxijet PHs are what i use in my tank.
to place them in the tank, use the flat nozzle that comes with the PHs and angle the flow so it ripples the surface. also, try to aim the flows so that they hit each other and create a random turbulence in the water.
you will have to adjust them a few times to prevent the flow from digging up your substrate, so dont get frustrated if you have a sand storm the first day. also, try to get flow in all areas of your tank, you dont want the have areas where the water doesnt circulate.