Only one backflow preventer between the water source and the valves is all that is needed. And in your case, since the water source isn't a drinking water source, you might not need any backflow preventer.
The only time you have one backflow preventer per circuit is when you are using anit-siphon values.
However, this is one area where you MUST check with the city building inspector to determine what are backflow requirements in YOUR area, especially for connecting to a non-potable water source. Some cities have VERY strict ordanances that required very specific backflows (down to manufacture part numbers are specified) and require certified plumbers to install and annually test backflows. Others are extreamly generic, something along the lines of "appropriate backflow must be installed".