how much coolant does a 2006 f-250 diesel hold

Your truck's coolant capacity is 27.5 quarts, or almost 7 gallons. Keep in mind that the coolant has many passages, including the EGR cooler and engine oil cooler.

It will be near impossible to drain ALL engine coolant from the system .. You will also need to use the correct mix of antifreeze and water for refilling .. Drain out as much coolant as you can - then refill the system with the appropriate coolant mixture...