I would want a minimum of 4 returns, 2 on each floor. They should be centrally and strategically located, preferably on inside hallways.

Normally, supplies are placed on or close to Ext walls and returns on interior walls.

For two floors and one system, I would want zoning controls with a separate thermostat for each floor/zone. If not that because of cost, you want damper controls to handle and balance supply air between the floors.

I would also recommend a good 4-5" pleated media cabinet conveniently located for easy changeout.

Just a few thoughts.

IMO

Thank you for your response tigerdunes. Very helpful. It sounds like we are having 2 returns (one on each floor) and it took a lot of configuring to figure out how to get the return to the ceiling of the second floor.

I am in a 1750sq ft ranch and have one in each bedroom the living room and a hallway going back to the laundry room... and one in the basement. I would want two on each floor minimum.

I talked to my builder and he says he trusts the HVAC guy since he is the expert here, but I'm still concerned because of the responses I've had here. He says since we are installing an extremely energy efficient heat pump & furnace that we only need one cold air exchange per floor and only one heat register per room (other than the 2 larger rooms where he recommends 2 each). Does this sound right?

"it took a lot of configuring to figure out how to get the return to the ceiling of the second floor."

High on a wall is usually adequate.

The extra 180 degree turn from a ceiling intake to the return duct is only going to restrict flow.

Here are the floor layouts if that helps. Bedrooms are on the bottom floor and main living space on the upper. I'm really stressing about this as I hope the HVAC guy isn't just trying to cut corners.

The efficiency of the system has nothing to do with how many returns you need. only size of the system and setup of the house... ie open floor plan vs a bunch of small rooms.
Having said that you talk like there is a problem with space to get return to second floor so if the return is the right size you will probably be ok with 1 on that floor due to it being pretty much an open floor plan.
Downstairs on the other hand it looks like 2 would be better.

On a side note I wouldn't have a house that is 2 floors without 2 systems..... I know it costs more but even with zoning People with 2 floors and 2 systems are more happy and live longer than people with 2 floors and 1 systems.