Not all engines are equal and that goes the same way for their ATF requirements and with this, the first thing you need to do is get the information from your owners manual. This will give you the information you need for the exact required liters for your particular vehicle. In the absence of the owner’s manual, check the dipstick as it will usually have markers for maximum and minimum fluid amount.
Also, you can smell and visually inspect the condition of the transmission fluid. If the fluid appears darker and smells burnt, that is usually a good indication that it needs a change. Combine these with good old common sense, such as how many miles has the car run and what kind of loads are you subjecting the car to. Hauling heavier loads translates to changing the fluids more often and at closer intervals.