Coal and charcoal usually allow 8 items to be cooked or smelted in a furnace or blast furnace. Since the problem with furnaces and blast furnaces not smelting ore was fixed coal and charcoal do not give the same results; in order to smelt 8 ore (or cook 8 fish) I have to put in 1 piece of coal or charcoal, and after it is used I have been placing in 1 stick for that extra little bit on the timer to finish off the last bit of ore (or fish); if I don't  the eighth item will not complete.