How much coolant nees to be drained for...

[JMBT]

5 gal would prolly do it if you can wiggle that section out once you back out those two banjos, but I would take out 10 gals just incase you also have to take the other ends of those two lines loose. Which you probably will because they are stiff. The one on your left finger connects to the actuator and the therm housing. The one on your right finger, goes to the after treatment Fuel doser.
Use carb cleaner not brake/parts cleaner as it will attract more soot even after it is dry. and a refrigerator brush, you can get it at walmart for 3 or 4 bucks. It’s about 2 ft long and 1.5” in diameter, made for reaching behind a fridge to clean the coils without pulling the fridge away from the wall. They work great for EGR tune ups.

If you truck is a Volvo, coolant drain is toward the bottom and back of the radiator on the drivers side, but you will need a special fitting to use it. Volvo sells the fitting on a short piece of hose for about $50. I can’t believe I am saying this, but it is actually worth the money vs. trying to unscrew the plug and then stick it back in without losing all your coolant.