Adding charcoal to a charcoal smoker is not a rocket science and has no specific set standard.it depends on the type of meat you are smoking and the technique by which you want to burn the fuel. In addition to that you can also decide it according to weather conditions. if the weather condition is normal then you can go with 8-10 lumps or already set quantity.  But if the weather is cold then you may need to add double the quantity of charcoal.