Installing Ductwork for Air Returns 

Yes you answered your question 😁 just one filter at the air handler

@@Skilledtradesman okay, do I add a filter box at the end of air handler, between ductboard

It depends on the airflow, the filter box position has to be before the air enters to your coils, you don’t want to clog up your coils with dust and stuff, on this video my air flow is going down so the filter box is up on top and the AC coils is at the bottom