do I need a pressure couple in a main water line?
We need more details. Your question is too vague.
"Pressure couple" is not a clear term ... Do you mean a pressure reducing valve? Answer: Yes. Otherwise, it is hard to help you ...
Pressure reducing valve is only needed if you have high city water pressure. Anything above 60-70 PSI should probably have one.
The code requires a PRV if your supply pressure is over 80 PSI