How many return air vents should a 1700 sq foot house have ?

I am thinking about replacing the central AC and heat system here at my house. I have a 5 year old house with one central 20 X 20 return. No basement or second floor. I have been told that I need to add 4 additional return air vents in the house. Should I do this along with changing out the central AC/furnace ?
The reason I am changing these items out is I currently have a 2.5 ton AC unit and it is way to small for the house especially when temps reach over 85 degrees. Going to either a 4 ton unit or 3.5 ton unit. Your thoughts ?