2 cups of mashed potato works out at about 1 pound (500g) of raw potatoes boiled and mashed. If you are making mash specifically for this recipe don't add any butter or cream to your mash.
Ideally you want to use mashed potatoes that haven't had butter and/or cream added to them. If you are using very wet/buttery mashed potatoes then you may need add extra flour to form a manageable dough.
Because there is no oil in the frying pan, the potato cakes don't get a golden color to them, instead they get a darker dirtier look. This is exactly what you are looking for and is completely traditional and authentic.