What pound thrust in a trolling motor would be needed for a 20 ft pontoon boat?

If it were me, I would purchase a 55 lb thrust motor and it would be a Motorguide. A 55 lb thrust motor will be a 12 volt system. Beyond 55 lb you'd need to move to a 24 volt system that would require two 12 volt batteries for the trolling motor. A 24 volt motor would offer around 80 to 90 lbs of thrust. Unless you're going to use your boat in heavy currents or strong winds, I'd opt for a 55 lb motor.