If the radiator on the vehicle is empty when you add coolant to the radiator it can take between 3 to 4 gallons of coolant to fill the radiator and engine block.
Larger radiators such as in a truck should take around 4 gallons of coolant and maybe a bit more coolant while smaller cars may only take 3 to 3 1/2 gallons of coolant.
Just have at least 4 gallons of coolant on hand when you fill the radiator and if you have some left you can keep it and top the coolant off if needed.
But 4 gallons is about right for the amount of gallons of coolant for most vehicles.